adjust deprecated alert box: no bottom margin

[osma]

Reasons for creating this PR

We agreed today (at the Finto weekly meeting) that alert boxes for deprecated concepts and vocab messages look better when the box is directly adjacent to the main content box. This PR implements the minor CSS adjustment for deprecated concept warnings.

Link to relevant issue(s), if any

• follow-up to Concept page: special display for deprecated concepts #1918 and Adjust warning message for deprecated concepts (See: X) and improve its accessibility #2001

Description of the changes in this PR

Remove the margin-bottom of the deprecated alert so it looks like this (notice no grey border between red and white areas):

Known problems or uncertainties in this PR

Bloats the CSS by one more line. I couldn't think of a cleaner solution...

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.42%. Comparing base (19f8723) to head (e764b4c).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #2026   +/-   ##
=========================================
  Coverage     70.42%   70.42%           
  Complexity     1703     1703           
=========================================
  Files            34       34           
  Lines          4436     4436           
=========================================
  Hits           3124     3124           
  Misses         1312     1312           

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