Update gcc leg from v15 to v16

[am11]

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[am11]

gcc leg picked up v16 and is green:

  Commencing build of "install" target in "CoreCLR component" for linux.x64.Checked in /__w/1/s/artifacts/obj/coreclr/linux.x64.Checked
  Invoking "/__w/1/s/eng/native/gen-buildsys.sh" "/__w/1/s/src/coreclr" "/__w/1/s/artifacts/obj/coreclr/linux.x64.Checked" x64 linux gcc Checked "ninja"  -DCLR_CMAKE_PGO_INSTRUMENT=0 -DCLR_CMAKE_OPTDATA_PATH= -DCLR_CMAKE_PGO_OPTIMIZE=0 -DCLI_CMAKE_FALLBACK_OS="linux" -DFEATURE_DISTRO_AGNOSTIC_SSL=1 -DFEATURE_DYNAMIC_CODE_COMPILED=1 -DCDAC_BUILD_TOOL_BINARY_PATH=/__w/1/s/artifacts/bin/coreclr/linux.x64.Checked/cdac-build-tool/cdac-build-tool.dll -DCLR_DOTNET_HOST_PATH='/__w/1/s/.dotnet/dotnet' -DCLR_DOTNET_RID=linux-x64 
  Not searching for unused variables given on the command line.
  -- The C compiler identification is GNU 16.1.0
  -- The CXX compiler identification is GNU 16.1.0
  -- Detecting C compiler ABI info
  -- Detecting C compiler ABI info - done
  -- Check for working C compiler: /usr/bin/gcc-16 - skipped
...

[am11]

@akoeplinger, @janvorli, this one is ready for review (gcc validation leg passed).