feat!: Support Django 4.2

[ankiaga]

Breaking changes

• Removes support for Django 2.2
• Transaction handling in Django 4.2 is different than in Django 3.2:
  • In Django 3.2: When Django tries to manually start a transaction while the dbapi connection is in AutoCommit=True mode, the Spanner Django provider would not start a transaction.
  • In Django 4.2: When Django wants to manually start a transaction while the dbapi connection is in AutoCommit=True mode, the Spanner Django provider will start a transaction.

Release notes

1. Adds support for Django 4.2
2. Removes support for Django 2.2
3. Only Python versions 3.8 and higher are supported
4. Adds support for transactions in AutoCommit=True mode by default for V4.2. Transactions in AutoCommit=True mode are by default not supported in prior versions (i.e. V3.2).
   • To enable manual transactions in AutoCommit=True mode in V3.2, please set ALLOW_TRANSACTIONS_IN_AUTO_COMMIT=True your settings.py file.
   • To disable transactions in AutoCommit=True mode in V4.2, please set ALLOW_TRANSACTIONS_IN_AUTO_COMMIT=False in your settings.py file.

[jaz-la]

thanks a lot for working on it, left a few comments. I'm hoping that we can also have a way to bypass the version check and fall into Django 4 logic with Django 5 at our own risk. Another suggestion is to add a summary in the PR description so new developers can understand the actual difference between django 3 vs 4 without digging into the code.