ImageBlock and Wagtail 6.4

What's Changed

• Add support for Wagtail 6.3 and 6.4 and Python 3.13 by @mgax in #416
• Add support for ImageBlock by @mgax in #419, with kudos to @b-tupiti for starting this in #411
• Remove support for Wagtail < 6.3 and Python 3.8 by @mgax in #416
• Updates to CI by @zerolab and @mgax in #415, #417, #418, #420, #421

Full Changelog: v0.27.0...v0.28.0