This fixes a few of the TODOs in the WORKSPACE file and changes a lot
about the popular_repos tests.

This commit also provides a default value of
`--java_runtime_version=remotejdk_21` just for the bazel_test in
lcov_coverage_test.go so that contributors who don't
have a JAVA_HOME on the host system can still get the
lcov_coverage_test.go to pass by default.

Oh, and it also fixes a TODO in the repository's `.bazelrc` file.

As a contributor to the project, I expect to be able to clone the
repository, go to the master branch and run `bazel test //...` and
have it work.

But, before this change, just like the CI system, contributors were
also expected to run `./tests/core/cgo/generate_imported_dylib.sh`
manually. Well ... no more!

That script should no longer be run.

The Ubuntu build is breaking because jdk21 is not supported on that
OS, and the Windows build is failing because some of the tools pprof
and diffp are not available on Windows.