Remove jQuery UI dependency

[osma]

Reasons for creating this PR

The jQuery Migrate plugins prints deprecation warnings about jQuery UI. We are currently using jQuery UI 1.12.1.

It would be possible to upgrade to jQuery UI 1.13.x, but it is not available via Packagist / Composer.

But it turns out we're not actually using any(?) functionality of jQuery UI. It was probably used in the past, but since then, UI widgets have been rewritten using other libraries. So this PR simply removes jQuery UI.

Link to relevant issue(s), if any

• related to Upgrading jQuery 2.x dependent libraries to support jQuery 3.x #1151

Description of the changes in this PR

• drop jQuery UI dependency in composer.json
• drop references to jQuery UI JS and CSS files from HTML templates

Known problems or uncertainties in this PR

Not 100% sure there isn't some functionality that is still based on jQuery UI that I just didn't find.

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)

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[codecov]

Codecov Report

Base: 71.96% // Head: 70.68% // Decreases project coverage by -1.28% ⚠️

Coverage data is based on head (1886101) compared to base (bb730b6).
Patch has no changes to coverable lines.

Additional details and impacted files

@@                   Coverage Diff                    @@
##             upgrade-jquery-3.6    #1365      +/-   ##
========================================================
- Coverage                 71.96%   70.68%   -1.29%     
+ Complexity                 1832     1646     -186     
========================================================
  Files                        32       32              
  Lines                      3966     3786     -180     
========================================================
- Hits                       2854     2676     -178     
+ Misses                     1112     1110       -2     

Impacted Files	Coverage Δ
model/LabelSkosXL.php	0.00% <0.00%> (-34.62%)	⬇️
model/Concept.php	83.63% <0.00%> (-4.01%)	⬇️
model/ConceptPropertyValue.php	86.25% <0.00%> (-0.81%)	⬇️
model/ConceptPropertyValueLiteral.php	95.65% <0.00%> (+1.77%)	⬆️

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[osma]

I've tested most UI functionality without jQuery UI and couldn't find anything broken.
I'll merge this into the upgrade-jquery-3.6 branch and hope that nothing breaks. Or if it does, we will notice it soon enough.