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]

#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.

[ChrisDodd]

There are a lot of new packages needed for the tofino backend that aren't mentioned in the dependencies list (ones I've run into: jsl, rapid-json, pyinstaller, jsonschema). Someone needs to start with a minimal install of each platform we want to support and figure out what packages are needed from there.

[fruffy]

There are a lot of new packages needed for the tofino backend that aren't mentioned in the dependencies list (ones I've run into: jsl, rapid-json, pyinstaller, jsonschema). Someone needs to start with a minimal install of each platform we want to support and figure out what packages are needed from there.

A particular problem with these dependencies is that some are Python binaries called in the CMake script. They actually cause the CMake configure steps to fail if they are not installed, which is annoying. Particularly with Ubuntu 24.04+ which requires the use of virtual Python environments.

Might be worthwhile to check whether some of these are still even required or can be vendored in some way.