chore: slim font size for section title in PR template

[legendecas]

Which problem is this PR solving?

Keep all section titles in the same font size.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

No tests.

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

Merging #2541 (9ba9fd5) into main (c160ad8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2541   +/-   ##
=======================================
  Coverage   93.05%   93.05%           
=======================================
  Files         139      139           
  Lines        5172     5172           
  Branches     1110     1110           
=======================================
  Hits         4813     4813           
  Misses        359      359