Update GitHub Actions

Update GitHub Actions

- Switch from ubuntu-20.04, which is not available anymore,
  to ubuntu-latest (currently Ubuntu 24.04)
- Update GitHub Actions:
  * actions/checkout from v4 to v6
  * codecov/codecov-action from v4 to v5
  * github/codeql-action/init from v3 to v4
  * github/codeql-action/analyze from v3 to v4
- Add workflow_dispatch to allow manual triggering, which simplifies
  testing of the following GitHub Actions:
  build-test-linux.yml, code-coverage.yml and codeql-analysis.yml.
- Update tests/functions for Python3 to avoid failing tests using
  `make check TESTS_STYLE="rrdcached-tcp"`
  Python3 uses socketserver, all lowercase. Python2 uses SocketServer.
  https://stackoverflow.com/a/66913346/5067752
- Update list of tests using valgrind-logfile
  Exclude currently failing tests (Ubuntu 24.04):
  graph1, graph2, rpn1, create-with-source-4, vformatter1