Add maintenance as release notes category

[sondreso]

Issue
Maintenance is an often used category, and many PRs currently does not fit well in other categories.

Approach
Add maintenance as category

Pre review checklist

• Added appropriate release note label
• PR title captures the intent of the changes, and is fitting for release notes.
• Commit history is consistent and clean, in line with the contribution guidelines.

Adding labels helps the maintainers when writing release notes. This is the list of release note labels.

[dafeda]

LGTM 👍

[codecov-commenter]

Codecov Report

Merging #3199 (73e37ae) into main (8248b97) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #3199   +/-   ##
=======================================
  Coverage   65.53%   65.53%           
=======================================
  Files         610      610           
  Lines       48891    48891           
  Branches     4392     4392           
=======================================
+ Hits        32041    32043    +2     
+ Misses      15384    15383    -1     
+ Partials     1466     1465    -1     

Impacted Files	Coverage Δ
libres/lib/enkf/block_fs_driver.cpp	81.25% <0.00%> (-0.57%)	⬇️
libres/lib/res_util/block_fs.cpp	53.02% <0.00%> (+0.33%)	⬆️
ert/data/record/_transformation.py	88.72% <0.00%> (+0.49%)	⬆️

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