feat: add support for protobuf <8.0.0

[jrandolf]

Summary

• Bumps the protobuf upper bound from <7.0.0 to <8.0.0 in both setup.py and .librarian/generator-input/setup.py
• All 1772 unit tests pass with both protobuf 6.x and 7.x

Test plan

• Ran tests/unit/ with protobuf 6.33.5 — 1772/1772 passed
• Ran tests/unit/ with protobuf 7.34.0 — 1772/1772 passed

[chalmerlowe]

📢 Migration Notice: 📢 This library is moving to the google-cloud-python monorepo soon.

We kept this PR open due to recent activity. We would like to finalize this PR so it can be merged if it is critical.

If we don't hear from the PR author, we will close this PR in the next few days. The PR can then be re-opened in the monorepo once the migration is complete and work can continue there.

[parthea]

As per #1586 (comment), this code is being migrated to google-cloud-python. We will be expanding the supported range for Protobuf in google-cloud-python for all packages in the after the code has migrated. Code migration is starting today.

[parthea]

Thanks @jrandolf for opening a PR. I created googleapis/google-cloud-python#16001 to follow up on this. After the code migrates to google-cloud-python, we will be using the output from the code generator in https://github.com/googleapis/gapic-generator-python for updating setup.py. If you're interested to submit a fix there, pull requests are welcome! The source of truth lives here: https://github.com/googleapis/gapic-generator-python/blob/535d65420751b5959f8e960ee532726da00159b6/gapic/templates/setup.py.j2#L45

Once the fix is merged in gapic-generator-python, it will be rolled out to google-cloud-python automatically using our tooling which is referred to as librarian.