[Backport 5.1] [permissions] Revert default behaviour for explicit permissions

[github-actions]

With the switch to the user_repo_permissions table, the default behaviour for explicit permissions changed. Previously, if explicit permissions API was enabled, then all repositories would be treated as restricted, regardless of code host connection configurations.

The user_repo_permissions table allows us to run permissions syncs and explicit permissions in parallel, but the default behaviour was to have code host connections be marked as unrestricted, unless authorization is enabled. However, not all code hosts support authorization, leading to connections that are always left unrestricted, even if explicit permissions are being used.

This PR reverts the default behaviour back to treating a repository as restricted if explicit permissions are enabled.

Test plan

Added unit tests for the new Unrestricted logic. Also tested locally with an "Other Git code host" connection.

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Backport 5ad2474 from #54419

[sourcegraph-bot]

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