[BUG] Change allowed_merge_methods to Optional

[deiga]

Resolves #3082

---

Before the change?

• allowed_merge_methods was set to Required and thus caused a breaking change

After the change?

• allowed_merge_methods is Optional as intended

Pull request checklist

• Schema migrations have been created if needed (example)
• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[github-actions]

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[stevehipwell]

LGTM

[faximan]

Did this fully fix the issue? We don't specify allowed_merge_methods, and while the apply is successful now, this diff keep coming back in every plan:

!           pull_request {
!               allowed_merge_methods             = [
-                   "merge",
-                   "rebase",
-                   "squash",
                ]

Using provider version 6.10.2.

[deiga]

@faximan I don't think it did. I've noticed as well that the plan needs the field to be set to show no changes...

[stevehipwell]

@deiga we're missing Computed: true.