Support other subclasses of `EntityManagerInterface` too

[NoelLH]

Description

Support any EntityManagerInterface in new generateId() methods, and also in existing generate()s.

Also require PHP 7.4+ and expand use of type constraints.

I suggest this targets a new major release.

Motivation and context

Detailed in https://github.com/thebiggive/matchbot/pull/338

How has this been tested?

Unit tests run and added to. Fork used in https://github.com/thebiggive/matchbot/pull/338 both with real local calls and unit tests.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)

I believe this is backwards incompatible only insofar as PHP version support, so I have not made any documentation changes or identified any that are needed.

PR checklist

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING.md document.
• [x] I have added tests to cover my changes.