Implement "auto" r_version value in backport_linter()

[Bisaloo]

Opening as a draft for a first general impression.

If this seems good, I still need to:

• Add a NEWS.md item
• Add tests

Should this be the new default value for backport_linter(r_version=)?

Fix #1974.

[codecov]

Codecov Report

❌ Patch coverage is 12.50000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.18%. Comparing base (dc8f149) to head (27bf933).

Files with missing lines	Patch %	Lines
R/settings_utils.R	0.00%	6 Missing ⚠️
R/backport_linter.R	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2907      +/-   ##
==========================================
- Coverage   99.28%   99.18%   -0.10%     
==========================================
  Files         128      128              
  Lines        7231     7239       +8     
==========================================
+ Hits         7179     7180       +1     
- Misses         52       59       +7     

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