[FEAT]: Support RepositoryMigration actor type in bypass_actors for ruleset

### Describe the need

The current implementation of `bypass_actors` in `github_repository_ruleset` and `github_organization_ruleset` resources does not support `RepositoryMigration` as a valid `actor_type`, causing the following error when applying Terraform configurations that include it:

expected actor_type to be one of ["Integration" "OrganizationAdmin" "RepositoryRole" "Team" "DeployKey"], got RepositoryMigration

This is needed when migrating repositories that already have rulesets configured - the `RepositoryMigration` actor type allows bypassing ruleset rules during migration, preventing failures when importing repositories with existing branch protection or ruleset configurations.

### SDK Version

_No response_

### API Version

_No response_

### Relevant log output

```shell
Error: expected actor_type to be one of ["Integration" "OrganizationAdmin" "RepositoryRole" "Team" "DeployKey"], got RepositoryMigration
```

### Code of Conduct

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