Add migration to remove region on downgrade

[tjkirch]

Older versions don't know about this setting and would fail to deserialize the
model if it's present. (We currently need to write a migration for any
added/removed settings, but this will be automated away.)

Region is a generated setting, so we're not throwing away anything important.

---

Fixes #642.
Builds on #638 and #644.

Testing done:

Saved contents to make sure we write everything out:

$ find datastore-sample/current/ > v0.1-contents

Migrate to v0.2. We can see the migration running and saying it has nothing to do.

$ cargo run -- --log-level trace --datastore-path datastore-sample/current --migration-directories ./migration-binaries --migrate-to-version 0.2
...
21:41:19 [DEBUG] (1) migrator: Sorted migrations: ["migrate_v0.2_remove-region"]
21:41:19 [ INFO] New data store is being built at work location /home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.2_Oq7CIMsaiEfnCNL3
21:41:19 [ INFO] Running migration command: "./migration-binaries/migrate_v0.2_remove-region" "--forward" "--source-datastore" "/home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.1_AmdROiQykcX0vQRy" "--target-datastore" "/home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.2_Oq7CIMsaiEfnCNL3"
21:41:19 [DEBUG] (1) migrator: Migration stdout: RemoveRegionMigration has no work to do on upgrade.
RemoveRegionMigration has no work to do on upgrade.

21:41:19 [DEBUG] (1) migrator: Migration stderr: 
21:41:19 [ INFO] Flipping /home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.2 to point to v0.2_Oq7CIMsaiEfnCNL3
21:41:19 [ INFO] Flipping /home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0 to point to v0.2

When v0.2 (of aws-k8s flavor) runs, it will set settings.aws.region:

$ mkdir datastore-sample/current/live/settings/aws && echo -n '"us-west-2"' > datastore-sample/current/live/settings/aws/region

Downgrade back to v0.1. We can see the migration running and removing the region. (It removes it from live settings; it wasn't set in pending, hence the second message.)

$ cargo run -- --log-level trace --datastore-path datastore-sample/current --migration-directories ./migration-binaries --migrate-to-version 0.1
...
21:42:30 [DEBUG] (1) migrator: Sorted migrations: ["migrate_v0.2_remove-region"]
21:42:30 [ INFO] New data store is being built at work location /home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.1_kZNHRvDkARjcHinw
21:42:30 [ INFO] Running migration command: "./migration-binaries/migrate_v0.2_remove-region" "--backward" "--source-datastore" "/home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.2_Oq7CIMsaiEfnCNL3" "--target-datastore" "/home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.1_kZNHRvDkARjcHinw"
21:42:30 [DEBUG] (1) migrator: Migration stdout: Removed settings.aws.region, which was set to '"us-west-2"'
Found no settings.aws.region to remove

21:42:30 [DEBUG] (1) migrator: Migration stderr: 
21:42:30 [ INFO] Flipping /home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.1 to point to v0.1_kZNHRvDkARjcHinw
21:42:30 [ INFO] Flipping /home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0 to point to v0.1

Region was removed:

$ ll datastore-sample/current/live/settings/aws/region
ls: cannot access 'datastore-sample/current/live/settings/aws/region': No such file or directory

...and so the file list is the same as before:

$ find datastore-sample/current/ > v0.1-contents-after
$ diff v0.1-contents{,-after}

[tjkirch]

This push rebases on #644 which only had an updated commit message.

[tjkirch]

Per the notes in #644, I need to modify this to actually remove the file from the filesystem, since the migrator in old versions (before #644 is merged) doesn't allow removal through the standard interface.

[tjkirch]

This push has no change in the implementation, it just rebases on the change from #644 that allows this migration to work even when downgrading to versions before the migrator interface change.

[tjkirch]

This push just rebases on the changes from #644.

[tjkirch]

This push is just a rebase onto the changes from #637.

[zmrow]

🔎

[tjkirch]

This push just rebases on a typo fix from #636.

[tjkirch]

This push is just a rebase onto changes from #637.

[tjkirch]

Just a rebase onto changes from #644.

[tjkirch]

This push was just a rebase onto changes from #644.

[tjkirch]

Rebase on develop; removed first-party exception from cargo-deny, and added migration to workspaces Cargo.toml and RELEASE.toml. (Conflict was with another migration added for 0.2.)