Use el8 binaries when we encounter RHEL9

[Robbybp]

Fixes #1456

Hopefully. I don't have RHEL9 or Rocky9, so I haven't tested.

Changes proposed in this PR:

• Change el9, rhel9, and rocky9 to point to our el8 binaries

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.35%. Comparing base (80d37a8) to head (dd20dd3).
Report is 32 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1457      +/-   ##
==========================================
- Coverage   76.36%   76.35%   -0.01%     
==========================================
  Files         394      394              
  Lines       64953    64953              
  Branches    14404    14404              
==========================================
- Hits        49601    49598       -3     
- Misses      12793    12797       +4     
+ Partials     2559     2558       -1     

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[lbianchi-lbl]

@eslickj just pinging you in case you can think of any reason why this change could have unexpected consequences; to me, without knowing the details, it looks reasonable.

[ksbeattie]

LGTM, waiting for @mrmundt to run testing from this branch

[mrmundt]

This PR is going to be superseded by IDAES/idaes-ext#274 <- completion of that will mean we have a RHEL9 binary anyway, so this temporary remapping will be irrelevant.

[ksbeattie]

Closed in preference to IDAES/idaes-ext#274