Simplify dealing with attnos during decompression

[antekresic]

We fetch key column attribute numbers from indexes on uncompressed chunks. These should not contain
system attribute numbers so there is no point in
adding/subtracting FirstLowInvalidHeapAttributeNumber.

Disable-check: force-changelog-file

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.14%. Comparing base (59f50f2) to head (e541dd8).
Report is 1009 commits behind head on main.

Files with missing lines	Patch %	Lines
tsl/src/compression/compression_dml.c	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7311      +/-   ##
==========================================
+ Coverage   80.06%   82.14%   +2.07%     
==========================================
  Files         190      217      +27     
  Lines       37181    38864    +1683     
  Branches     9450    10025     +575     
==========================================
+ Hits        29770    31924    +2154     
+ Misses       2997     2959      -38     
+ Partials     4414     3981     -433     

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