Release 1.9.0

[Art4]

In this PR I will prepare everything for the release of SimplePie 1.9.0

Check list:

• Merge of Add support for PSR-18 HTTP clients #826
• Merge of SimplePie: Prepare PubSubHubbub for PSR-18 clients #838
• Merge of Stop legacy file cache constantly updating #846
• Merge of Enable test execution with PHPUnit 10 #848
• Merge of PHPStan level 8 #857
• bump version in constant SimplePie\SimplePie::VERSION.
• Add entries for all changes in CHANGLOG.md.
• Add missing authors and issues in CHANGLOG.md if necessary.
• Fix code style by running bin/php-cs-fixer fix.

We've already made great progress on the master branch. The last big issue on our roadmap (#731) is: "deprecate the SimplePie\File class (provide HTTP client instead)".

However this issue requires also the deprecation of the Sniffer and Locator classes. This requires another work tracked in #849. I propose to move this issue into the next minor release (1.10.0) and all planned issues for 1.10.0 should be moved to 1.11.0 on the roadmap.

• 1.9.0: Allow external HTTP libraries
• 1.10.0: deprecate File, Locator, Sniffer classes and provide alternatives; add option to throw or mute deprecation warnings
• 1.11.0: trigger deprecation warnings for a smoother upgrade path
• 2.0.0: remove deprecated code

[jtojnar]

We will still want to raise the PHPStan level to at least 8.