chore: disable required linear history on v3 branch

[tswast]

This should prevent conflicts when we sync v3 with main.

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[tswast]

Seems I got the schema wrong. I was guessing the property based on required_linear_history here: https://docs.github.com/en/rest/reference/repos#update-branch-protection

[busunkim96]

@tswast There's an example config with comments on all the possible options in the bot README: https://github.com/googleapis/repo-automation-bots/tree/main/packages/sync-repo-settings

https://github.com/googleapis/python-aiplatform/blob/5b1326f2516a0992afe3dab39789889bdbb12668/.github/sync-repo-settings.yaml#L3

mergeCommitAllowed: true should work.

[tswast]

Hmmm... Looks like sync-repo-settings config doesn't allow a per-branch setting for mergeCommitAllowed, but maybe I do still need to set it.

https://github.com/googleapis/repo-automation-bots/blob/9ca793a0420286600bb075e23430d594131ba34b/packages/sync-repo-settings/src/sync-repo-settings.ts#L277

requiresLinearHistory seems to be the right name. Not sure why it didn't match the spelling of the GitHub API.

https://github.com/googleapis/repo-automation-bots/blob/9ca793a0420286600bb075e23430d594131ba34b/packages/sync-repo-settings/src/sync-repo-settings.ts#L183

I think I need both.