Update the dependencies list in README & what platforms do we want to support

[vlstill]

This could be done in step with #4347, but we should decide what platforms/versions we should actually support. This list is now very conservative and the testing is even more (testing that Ubuntu 18.04 works!). I'm not sure if we have any rules on how to drop support for platforms (e.g. for Ubuntu 18). Any changes should be made in sync with what we actually test.

• clang 6 is never tested as far as I see, we test Clang 10 in the sanitizers builds
• GCC 9.1 is also never tested, we test 9.4 in both Ubuntu 20 and Ubuntu 18 builds -- this is probably less of a problem than Clang 6, but there could still be some bugfixes
• BWGC is no longer required (to be installed beforehand)
• protobuf is no longer required (to be installed beforehand)
• we should mention macOS for M* CPUs

Somewhat related is that I think we should add Ubuntu 24.04 to tests and both 22.04 and 24.04 to the list of supported targets.

[fruffy]

https://github.com/p4lang/p4c/issues/3986 is related.

Maybe there is a way to improve the Github action workflow to reuse components that have been build. This way we can instantiate a more complex test matrix while saving resources.

Not sure how much is too much, the breadth of testing can already be overwhelming but is required for a compiler that is used in many different environments.