18.3.0-beta1

What's Changed

• Bump phpstan/phpstan-doctrine from 1.3.14 to 1.3.15 by @dependabot in #1924
• Bump phpstan/phpstan-doctrine from 1.3.15 to 1.3.16 by @dependabot in #1928
• Bump phpstan/phpstan from 1.8.7 to 1.8.8 by @dependabot in #1929
• Implement purgeUnread setting by @Grotax in #1931
• Add some documentation about the integration tests by @Grotax in #1882
• Release 18.3.0-beta1 by @Grotax in #1933

Full Changelog: 18.2.0...18.3.0-beta1