adding pdg code for eta meson

[fjonasALICE]

added missing Pdg code for eta meson (221), which is also not included in root TPDGCode

[github-actions]

REQUEST FOR PRODUCTION RELEASES:
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The following labels are available
async-2023-pbpb-apass4
async-2023-pp-apass4
async-2024-pp-apass1
async-2022-pp-apass7
async-2024-pp-cpass0
async-2024-PbPb-apass1
async-2024-ppRef-apass1
async-2024-PbPb-apass2
async-2023-PbPb-apass5

[fjonasALICE]

The O2Physics task /PWGHF/HFL/TableProducer/electronSelectionWithTpcEmcal.cxx seems to be defining kEta itself without a proper namespace. This PR should be on hold and kEta moved to its own namespace in that task, which then can get adjusted to the central pdg code...

[alibuild]

Error while checking build/O2/fullCI_slc9 for 1711d0d at 2025-06-26 17:20:

## sw/BUILD/O2Physics-latest/log
/sw/SOURCES/O2Physics/14448-slc9_x86-64/0/PWGHF/HFL/TableProducer/electronSelectionWithTpcEmcal.cxx:562:45: error: reference to 'kEta' is ambiguous
/sw/SOURCES/O2Physics/14448-slc9_x86-64/0/PWGHF/HFL/TableProducer/electronSelectionWithTpcEmcal.cxx:572:51: error: reference to 'kEta' is ambiguous
/sw/SOURCES/O2Physics/14448-slc9_x86-64/0/PWGHF/HFL/TableProducer/electronSelectionWithTpcEmcal.cxx:580:54: error: reference to 'kEta' is ambiguous
/sw/SOURCES/O2Physics/14448-slc9_x86-64/0/PWGHF/HFL/TableProducer/electronSelectionWithTpcEmcal.cxx:587:54: error: reference to 'kEta' is ambiguous
/sw/SOURCES/O2Physics/14448-slc9_x86-64/0/PWGHF/HFL/TableProducer/electronSelectionWithTpcEmcal.cxx:594:57: error: reference to 'kEta' is ambiguous
ninja: build stopped: subcommand failed.

Full log here.