chore: bump tested up to WordPress 7.1

[erseco]

Summary

• Update Tested up to in readme.txt from 7.0 to 7.1 so plugin-check (and the WordPress.org listing) treats the plugin as compatible with the current WordPress version.

WordPress 7.1 is current; Plugin Check fails CI with outdated_tested_upto_header until this header matches.

Test plan

• CI make check-plugin passes without the outdated_tested_upto_header error.

[github-actions]

Test in WordPress Playground

Test the plugin with the code from this branch:

ℹ️ The eXeLearning editor is fetched from the shared release and unpacked into the plugin when the playground boots, so the first load may take a few extra seconds. ELP upload, shortcode, Gutenberg block and preview work normally.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.87%. Comparing base (e387742) to head (ce42a7e).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #107   +/-   ##
=========================================
  Coverage     96.87%   96.87%           
  Complexity      864      864           
=========================================
  Files            39       39           
  Lines          4323     4323           
=========================================
  Hits           4188     4188           
  Misses          135      135           

Flag	Coverage Δ
javascript	95.70% <ø> (ø)
php	97.25% <ø> (ø)

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