Make doctrine/annotations optional + drop PHP < 8.1

[W0rma]

Follow-up of willdurand/Hateoas#334

[W0rma]

Jobs with symfony ^3.0 and ^4.0 fail if doctrine/annotations package is not installed.
Looks like this is because jms/serializer-bundle:4.2 does not really work without this package.

Tests with serializer-bundle v3 and v5 look fine.

@goetas Do you think it's worth the effort to look into the issue with v4.2 or is it sufficient to have the tests without annotations only for symfony ^5.0 and higher?

[goetas]

Running tests on most recent symfony versions is more than acceptable.

I would not mind dropping old php and symfony versions as well (if it helps you here)

[goetas]

Hi, thanks a lot, I will check it later in the evening

[W0rma]

Tests are green.

All supported symfony versions are still tested.
The tests without doctrine/annotations package are only executed with symfony 5 and higher.

I tested the code in my personal project and it worked fine.
Please note that I neither use attributes nor annotations in my project.

[goetas]

I could not spot anything wrong, great work! thank you!