CI: add GHC 8.6.5 to validate-old-ghcs

[ulysses4ever]

fix #11073

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Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

[mergify]

This pull request has been removed from the queue for the following reason: checks failed.

The merge conditions cannot be satisfied due to failing checks:

• ❌ Validate post job

You can check the last failing draft PR here: #11079.

You may have to fix your CI before adding the pull request to the queue again.
If you update this pull request, to fix the CI, it will automatically be requeued once the queue conditions match again.
If you think this was a flaky issue instead, you can requeue the pull request, without updating it, by posting a @mergifyio requeue comment.