devel/pmd: Update to version 7.21.0
Release notes:
https://docs.pmd-code.org/pmd-doc-7.21.0/pmd_release_notes.html
https://docs.pmd-code.org/pmd-doc-7.20.0/pmd_release_notes.html
https://docs.pmd-code.org/pmd-doc-7.19.0/pmd_release_notes.html
https://docs.pmd-code.org/pmd-doc-7.18.0/pmd_release_notes.html
https://docs.pmd-code.org/pmd-doc-7.17.0/pmd_release_notes.html
Approved by: eduardo (mentor, implicit)
MFH: 2026Q1
(cherry picked from commit 59c87d2966be74127ce00dade032e7a6cd6ec9f0)