Wagtail >= 5.0 upgrade

[katdom13]

Wagtail 5.1 (See release notes)

• Update testing matrix to include Wagtail 5.1
• Drop support for Python 3.7

Wagtail 5.2 (See release notes)

• Add Wagtail 5.2 to test matrix
• Drop tests for Wagtail 4.2 and 5.0 as they have reached EOL

[jams2]

Thanks @katdom13 and @lparsons396 - I'll include these changes in #84