Wagtail 74 maintenance

[nickmoreton]

Summary

Maintenance update aligning the Wagtail support range with the current release schedule.

• Raise minimum to wagtail>=7.0; drop EOL Wagtail 6.3 LTS and 7.2 from install_requires, classifiers, and CI
• Add Wagtail 7.4 LTS (released 2026-05-04) to the CI matrix
• Keep Django 4.2 LTS — still supported by Wagtail 7.0 LTS and 7.3
• Exclude the Wagtail 7.4 + Django 4.2 combination (7.4 dropped Django 4.2)
• Remove stale WAGTAIL_VERSION >= (6, 0) test guard now that the minimum is 7.0
• Update README.md and docs/1_getting_started.md to state the new Wagtail 7.0+ minimum
• Refresh the PolyfillColorPanel feature blurb to note its deprecation
• Record the additions and EOL drops in CHANGELOG.md under [Unreleased]

Resulting supported matrix: Python 3.10–3.14, Django 4.2/5.2/6.0, Wagtail 7.0/7.3/7.4 (with Wagtail 7.1/7.2 excluded as EOL).

The wagtail.admin.telepath vs wagtail.telepath guard in widgets.py is intentionally retained because Wagtail 7.0 LTS remains in the supported range.

Test plan

• CI matrix passes across all Python × Django × Wagtail combinations
• Local python runtests.py passes against Wagtail 7.4 + Django 6.0 + Python 3.13
• Local python runtests.py passes against Wagtail 7.0 + Django 4.2 + Python 3.10
• black --check and isort --check-only pass

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