chore: bump compatibility headers

[justlevine]

What

Why

How

Testing Instructions

Additional Info

Checklist:

• This PR is tested to the best of my abilities.
• This PR follows the WordPress Coding Standards.
• This PR has proper inline documentation.
• This PR has unit tests to verify the code works as intended.
• The changes in this PR have been noted in CHANGELOG.md

[Copilot]

Pull Request Overview

This pull request updates compatibility headers and dependency versions to support newer versions of WordPress and WPGraphQL.

• Bump "Tested up to" in wp-graphql-gravity-forms.php and readme.txt from WordPress 6.6 to 6.8
• Update dependency versions in composer.json for wp-graphql-testcase and wp-graphql-stubs
• Adjust integration testing workflow to include additional WordPress versions and update actions versions

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
wp-graphql-gravity-forms.php	Updated compatibility headers for WP and WPGraphQL
readme.txt	Updated testing compatibility information
phpstan.neon.dist	Removed an ignored error pattern
composer.json	Bumped dependency versions for wp-graphql-testcase and wp-graphql-stubs
CHANGELOG.md	Documented new testing compatibility changes
.github/workflows/integration-testing.yml	Expanded WordPress matrix and updated PHP/setup and CodeClimate actions

[coveralls]

Pull Request Test Coverage Report for Build 14954921963

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 84.041%

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Totals
Change from base Build 10223145370:	0.0%
Covered Lines:	6393
Relevant Lines:	7607

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💛 - Coveralls