Upgrade to core EPUBCheck 5.2.1

[clapierre]

The current version of the EPUBCheck engine is at the old 5.1.0, there were a number of fixes in the updated 5.2.1 version, updated Jan 14, 2025 (Supports EPUB 3.3 and Accessibility 1.1.)
Thanks

[jonaslil]

Now EPUBCheck 5.3.0 has been released (1 September 2025), which underscores the need for an update of the engine.