Remove mappings from el9 to ubuntu

[mrmundt]

Fixes #1456

Summary/Motivation:

The infrastructure is already nicely defined in get-extensions to yell loudly / stop if someone is on an unrecognized platform or a platform not seen in the distribution map. We discussed in #1456 that we should not make an arbitrary mapping and instead tell the user to use the --distro option. This removes those arbitrary mappings, so now anyone who tries to run get-extensions on something we have not released will be given an error plus a help message.

Changes proposed in this PR:

• Remove distributions that we do not officially support yet

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.85%. Comparing base (02cd0e8) to head (4266427).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1599      +/-   ##
==========================================
- Coverage   76.85%   76.85%   -0.01%     
==========================================
  Files         394      394              
  Lines       63241    63241              
  Branches    10359    10359              
==========================================
- Hits        48604    48601       -3     
- Misses      12188    12192       +4     
+ Partials     2449     2448       -1     

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[ksbeattie]

LGTM