Renaming Database3 to Database, but creating API stubs

[john-science]

What is the change?

This PR nominally renames Database3 to Database. But, since that is a big API-breaking change, this PR also creates API stubs for database3.py to fully support the old name.

Why is the change being made?

Database3 was only ever meant to be a placeholder name, while ARMI made the switch from the old Database class to the new one. The name was supposed to be fixed ~6 years ago.

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes have been updated if necessary.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.

[john-science]

Testing in all the downstream projects I can find, this is a pretty seamless change.

Since I preserved Database3 in the API, nearly everything works perfectly.

I only found one place where someone was doing a mock of Database3 directly in a unit test in a place where Database would be the new default. So, that one, ultra-specific unit test will need a one-line update.

[john-science]

This was about #1673

[john-science]

Too much time has passed, I will try to get to this PR again in August/September.

I still think this is a good idea.