[google-django-spanner][post-migration] Fix failing mono-repo presubmits and block releases

[ohmayr]

Presubmits for google-django-spanner are currently failing following the mono-repo migration. This issue serves as a strict release blocker to prevent new versions from being published until all CI/CD checks are restored to a passing state.

[parthea]

Tests are passing as of #16624 but they are flaky. The work to eliminate the flaky tests will be tracked in #16839