base-builder image uses Ubuntu 20.04, new version with Ubuntu 24.04 needed for fuzzing expat

[hartwork]

Hi!

Context

The build of libexpat started failing after I started using libprotobuf-mutator in the build system (since pull request libexpat/libexpat#950) for a new fuzzer named xml_lpm_fuzzer. The apparent CMake configure issue about abseil-cpp not being present I fixed upstream via libexpat/libexpat#952 for good but beyond that libprotobuf-mutator is not made for Ubuntu this old by any means, it even needs fixes to compile on Ubuntu 24.04 without problems, see my related (yet to review and merge) pull request google/libprotobuf-mutator#263 of yesterday. So my pull request #13015 here seems doomed to fail with no good options forward.

Need

While there are other fuzzers in here with workarounds to make libprotobuf-mutator work (e.g. through -DLIB_PROTO_MUTATOR_DOWNLOAD_PROTOBUF=ON) it's not fun in terms of runtime, and not something I'm willing to do in the Expat upstream build system (..); without Ubuntu 24.04 in OSS-Fuzz (a) I will just have to disable fuzzer xml_lpm_fuzzer upstream for builds with -DEXPAT_OSSFUZZ_BUILD=ON to fix the OSS-Fuzz build or (b) we're back to you maintaining more for Expat here when the original mission was to move as much fuzzing upstream as possible. Unless you have other non-hack ideas.

What do you think?

[maflcko]

Related: #11835

[hartwork]

Update: The related issue has received an automated reminder today at https://issues.oss-fuzz.com/issues/394367616#comment3 so I'm disabling compilation of fuzzer xml_lpm_fuzzer upstream now (libexpat/libexpat#955) while OSS-Fuzz images are too old to compile successfully in order to resurrect the other fuzzers. Keeping the ticket open so that xml_lpm_fuzzer has a chance of coming back…