Wagtail 4.0+ Upgrades

[katdom13]

Includes changes from this PR: https://github.com/wagtail/wagtail-autocomplete/pull/156

Upgrade related fixes:

• Includes 4.0, 4.1, 4.2, 5.0, and 5.1 upgrade considerations
• Removed Wagtail upperbound
• Update test matrix to include Wagtail 5.2
• Remove tests for Wagtail 4.2 and 5.0 as they have reached their EOL

Other fixes:

• Configure github workflow for testing and use tox-gh-actions

[SaptakS]

I have created https://github.com/wagtail/wagtail-autocomplete/pull/172 after resolving the merge conflicts. Closing this.