patch FxFx LHE to include PDF4LHC21 weights [skip modfiles]

Jan Kretzschmarrequested to merge
jkretz_patchFxFx_PDF4LHC21 into master

Description of bug

ATLMCPROD-9875

Last-minute addition of PDF4LHC21 weights tested locally, runs fine

Changes introduced

Tests

Issues resolved

Closes #

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