Add a mapping for Pop!_OS (default for System76 laptops) to get-extensions

[dangunter]

Fixes

Summary/Motivation:

Pop!_OS is based on Ubuntu which is supported, but without this mapping idaes get-extensions fails.

Changes proposed in this PR:

• Add a mapping from current version of Pop!_OS to associated version of Ubuntu

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.04%. Comparing base (5841433) to head (0255d3d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1534      +/-   ##
==========================================
- Coverage   77.05%   77.04%   -0.01%     
==========================================
  Files         384      384              
  Lines       62338    62338              
  Branches    10222    10222              
==========================================
- Hits        48034    48031       -3     
- Misses      11875    11879       +4     
+ Partials     2429     2428       -1     

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[ksbeattie]

LGTM