Upgrade EasyRdf to fix Turtle fragment URIs

[osma]

Reasons for creating this PR

We were using EasyRdf 1.7.x which is quite old and produces invalid Turtle fragment URIs in some cases, see #1591. The fix is to upgrade to easyrdf 1.10.x or newer. This PR upgrades to EasyRdf 1.13.x which is the most recent release that still works without breaking Skosmos 2 PHPUnit tests; EasyRdf 1.14.1 is currently the newest release. I don't think it makes sense to do extensive debugging of the failing Skosmos 2 unit tests, so I'm leaving the dependency at 1.13.x.

We need to drop support for PHP 7.x because EasyRdf 1.10 and above require at least PHP 8.0. This leaves only two supported PHP versions for Skosmos 2: PHP 8.0 and 8.1.

Link to relevant issue(s), if any

• Fixes Turtle output with fragment URIs is invalid #1591 (for skosmos-2)

Description of the changes in this PR

• upgrade to sweetrdf/easyrdf 1.13.x
• drop PHP 7.3 and 7.4 from the GitHub Actions CI test because they're incompatible with recent EasyRdf versions
• update README section on supported versions

Known problems or uncertainties in this PR

I didn't investigate closer why some PHPUnit tests fail when using easyrdf 1.14.x.

We need to do a similar EasyRdf upgrade for Skosmos 3 as well (main branch).

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]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.65%. Comparing base (7a53972) to head (a9d010b).
Report is 28 commits behind head on skosmos-2.

Additional details and impacted files

@@               Coverage Diff               @@
##             skosmos-2    #1671      +/-   ##
===============================================
- Coverage        70.33%   69.65%   -0.68%     
- Complexity        1669     1784     +115     
===============================================
  Files               32       32              
  Lines             4305     4581     +276     
===============================================
+ Hits              3028     3191     +163     
- Misses            1277     1390     +113     

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[joelit]

LGTM