Bump versions after 3.8 deprecation

[gareth-ellis]

Now rally needs 3.9 or later, we need to bump versions in rally-tracks system tests

[github-actions]

💔 All backports failed

Status	Branch	Result
❌	8.15	An unhandled error occurred. Please see the logs for details

Manual backport

To create the backport manually run:

backport --pr 729

Questions ?

Please refer to the Backport tool documentation and see the Github Action logs for details

[github-actions]

💔 All backports failed

Status	Branch	Result
❌	8.15	An unhandled error occurred. Please see the logs for details

Manual backport

To create the backport manually run:

backport --pr 729

Questions ?

Please refer to the Backport tool documentation and see the Github Action logs for details

[gareth-ellis]

💚 All backports created successfully

Status	Branch	Result
✅	8.15

Note: Successful backport PRs will be merged automatically after passing CI.

Questions ?

Please refer to the Backport tool documentation

[esbenchmachine]

@gareth-ellis
A backport is pending for this PR. Please add all required vX.Y version labels.

• If it is intended for the current Elasticsearch release version, apply the corresponding version label.
• If it also supports past released versions, add those labels too.
• If it only targets a future version, wait until that version label exists and then add it.
  (Each rally-tracks version label is created during the feature freeze of a new Elasticsearch branch).

Backporting entails:

1. Ensure the correct version labels exist in this PR.
2. Ensure backport PRs have backport label and are passing tests.
3. Merge backport PRs (you can approve yourself and enable auto-merge).
4. Remove backport pending label from this PR once all backport PRs are merged.

Thank you!