Refresh plugin metadata references

[RichardGeorgeDavis]

Closes #101

Summary

• remove stale WordPress.org plugin badges from README.md
• update the plugin URI to the canonical GitHub repository
• align the runtime plugin version constant with the current plugin header version
• update readme.txt compatibility metadata to WordPress 6.9.4
• note in README.md that official release changelog entries belong in readme.txt

Verification

• php -l divi-accessibility.php

Notes

• this PR intentionally does not touch package.json because old open PR Update package.json version number #84 already covers that exact version bump
• current WordPress release confirmed via the official version-check API: https://api.wordpress.org/core/version-check/1.7/
• old WordPress.org plugin page closure confirmed here: https://wordpress.org/plugins/accessible-divi/?output_format=md

[RichardGeorgeDavis]

Superseded by #121, which consolidates this work into the 2.1.0 release candidate branch.