[draft] expat: Fix build

[hartwork]

Gets me compile errors…

/src/expat/build/libprotobuf-mutator/src/libprotobuf-mutator/src/mutator.cc:90:41: error: no member named 'has_presence' in 'google::protobuf::FieldDescriptor'
   90 |   return !field.is_repeated() && !field.has_presence();
      |                                   ~~~~~ ^
[ 48%] Linking C executable tests/runtests
/src/expat/build/libprotobuf-mutator/src/libprotobuf-mutator/src/mutator.cc:383:15: error: call to non-static member function without an object argument
  383 |   return Any::GetDescriptor() == message->GetDescriptor()
      |          ~~~~~^~~~~~~~~~~~~
/src/expat/build/libprotobuf-mutator/src/libprotobuf-mutator/src/mutator.cc:389:15: error: call to non-static member function without an object argument
  389 |   return Any::GetDescriptor() == message->GetDescriptor()
      |          ~~~~~^~~~~~~~~~~~~
/src/expat/build/libprotobuf-mutator/src/libprotobuf-mutator/src/text_format.cc:31:10: error: no member named 'SetRecursionLimit' in 'google::protobuf::TextFormat::Parser'
   31 |   parser.SetRecursionLimit(100);
      |   ~~~~~~ ^
/src/expat/build/libprotobuf-mutator/src/libprotobuf-mutator/src/text_format.cc:33:10: error: no member named 'AllowUnknownField' in 'google::protobuf::TextFormat::Parser'
   33 |   parser.AllowUnknownField(true);
      |   ~~~~~~ ^

…due to too-old libprotobuf-dev 3.6.1.3-2ubuntu5.2 in Ubuntu 20.04.

Seems doomed to fail with with no good options forward, see #13016 for the big picture.

[github-actions]

hartwork is either the primary contact or is in the CCs list of projects/expat.
hartwork has previously contributed to projects/expat. The previous PR was #4148