Avoid showing vocabulary modified timestamp on concept page when the concept doesn't have a specific modification date

[osma]

Reasons for creating this PR

See #1343

Link to relevant issue(s), if any

• Closes Skosmos shouldn't show the modified timestamp for a concept page unless such is explicitly stated for the concept data #1343

Description of the changes in this PR

• introduce a new method Concept.getConceptModifiedDate that only returns the specific modification date of the concept but doesn't fall back to the vocabulary-level modified timestamp
• use getConceptModifiedDate when showing modified timestamps on the concept page
• unit test to verify

Known problems or uncertainties in this PR

Some unit tests will fail with deprecation warnings on PHP 8.1 for unrelated reasons - see PR #1567 for the explanation and fix.

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)

[sonarqubecloud]

SonarCloud Quality Gate failed.   

0 Bugs
0 Vulnerabilities
1 Security Hotspot
0 Code Smells

No Coverage information
0.0% Duplication

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3b9489a) 70.22% compared to head (9c085eb) 70.23%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1568      +/-   ##
============================================
+ Coverage     70.22%   70.23%   +0.01%     
- Complexity     1665     1667       +2     
============================================
  Files            32       32              
  Lines          4295     4300       +5     
============================================
+ Hits           3016     3020       +4     
- Misses         1279     1280       +1     

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[joelit]

Looks good, works in my environment