[Updater] Extract RefreshVersionPullRequest as an Operation class

[brrygrdn]

Follows on from #6866, #6884

This branch peels out the Updater code required to accept a job which is "refreshing" an existing Dependabot PR for a version update using the same approach as before:

• Extract the entire code path required from the Updater
• Remove any code related to non-concerns for this operation type, in this case security-updates primarily
• Wire it into the Updater to be used for a specific job type, verifying current testing passes

Why Refresh?

We use different terminology for this operation in some places depending on the trigger, but it isn't correct to think of it as a "rebase" despite commands like @dependabot rebase existing.

The actual behaviour is one of three outcomes:

• If the project still needs the same dependency at the same version, then the PR is updates on top of the current head of the target branch; this is effectively a rebase
• If the project still needs the same dependency updated, but there is a newer version, then a new PR is created to supersede the existing one
• If a number of other circumstances have changed, such as the Dependency now being removed, ignored, updated by another commit, etc, then the PR is closed.

The term "update" is fairly saturated in Dependabot for obvious reasons, so I went with "refresh" as a good-enough catch all term.

Notes

As with #6866, this code is intentionally not DRY'd out as much as possible. We've de-risked this approach and this branch completes the rule of three, so the next step will be push the shared code into an interface mixin that is common to all Operation classes before starting to peel out the two Security Operations.

This PR is also branched off #6934 which should ship first as an enabling change.

[brrygrdn]

@landongrindheim I've added a last commit removing some dead code from the def dependencies method for the new operation - this operation cannot be invoked if job.dependencies is empty, so we will always early return the outcome of the dependency_snapshot.dependencies.select statement.