Add JDK21 to supported platforms page

[gdams]

Also add Alpine Linux aarch64

Description of change

Checklist

• npm test passes
• documentation is changed or added (if applicable)
• permission has been obtained to add new logo (if applicable)
• contribution guidelines followed here

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[codecov]

Codecov Report

Merging #2317 (2eab546) into main (f4039c4) will not change coverage.
Report is 20 commits behind head on main.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2317   +/-   ##
=======================================
  Coverage   99.20%   99.20%           
=======================================
  Files          87       87           
  Lines        6542     6542           
  Branches      552      552           
=======================================
  Hits         6490     6490           
  Misses         52       52           

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[steelhead31]

LGTM