GH-50412: [CI][GLib][Ruby] Remove some unnecessary Ubuntu 20.04 cases

Author: raulcd

c_glib/doc/meson.build

@@ -22,6 +22,7 @@ gi_docgen_toml_conf = configuration_data()
 gi_docgen_toml_conf.set('SOURCE_REFERENCE', source_reference)
 # We can't use "version.replace('-SNAPSHOT', '.dev')" here because
 # Ubuntu 20.04's Meson is < 0.58.0.
+# Is the above comment still relevant?
 if version_tag == ''
     gi_docgen_version_tag = ''
 else

ci/scripts/cpp_build.sh

@@ -157,10 +157,6 @@ if [ "${ARROW_USE_MESON:-OFF}" = "ON" ]; then
   CC="${ORIGINAL_CC}"
   CXX="${ORIGINAL_CXX}"
 elif [ "${ARROW_EMSCRIPTEN:-OFF}" = "ON" ]; then
-  if [ "${UBUNTU}" = "20.04" ]; then
-    echo "arrow emscripten build is not supported on Ubuntu 20.04, run with UBUNTU=22.04"
-    exit -1
-  fi
   n_jobs=2 # Emscripten build fails on docker unless this is set really low
   source ~/emsdk/emsdk_env.sh
   export CMAKE_INSTALL_PREFIX=$(em-config CACHE)/sysroot

ruby/red-arrow/ext/arrow/extconf.rb

@@ -62,13 +62,6 @@
   MESSAGE
 end
 
-# Old re2.pc (e.g. re2.pc on Ubuntu 20.04) may add -std=c++11. It
-# causes a build error because Apache Arrow C++ requires C++17 or
-# later.
-#
-# We can remove this when we drop support for Ubuntu 20.04.
-$CXXFLAGS.gsub!("-std=c++11", "")
-
 [
   ["glib2", "ext/glib2"],
 ].each do |name, relative_source_dir|