Update to PSR1-2 in docs (#5132)

CONTRIBUTING.md

@@ -6,7 +6,6 @@ Drush is built by people like you! Please [join us](https://github.com/drush-ops
 ## Git and Pull requests
 * Contributions are submitted, reviewed, and accepted using GitHub pull requests.
 * The latest changes are in the `11.x` branch. PR's should initially target this branch.
-* Try to make clean commits that are easily readable (including descriptive commit messages!)
 * See the test-specific [README.md](https://github.com/drush-ops/drush/blob/11.x/docs/contribute/unish.md) for instructions on running the test suite. Test before you push. Get familiar with Unish, our test suite. Optionally run tests in the provided Docker containers.
 * We maintain branches named 11.x, 10.x, etc. These are release branches. From these branches, we make new tags for patch and minor versions.
 
@@ -16,7 +15,7 @@ Drush is built by people like you! Please [join us](https://github.com/drush-ops
 
 ## Coding style
 * Do write comments. You don't have to comment every line, but if you come up with something that's a bit complex/weird, just leave a comment. Bear in mind that you will probably leave the project at some point and that other people will read your code. Undocumented huge amounts of code are nearly worthless!
-* We use [PSR-2](https://www.php-fig.org/psr/psr-2/) in the /src directory. [Drupal's coding standards](https://drupal.org/coding-standards) are still used in the includes directory (deprecated code).
+* We use [PSR-12](https://www.php-fig.org/psr/psr-12/).
 * Keep it compatible. Do not introduce changes to the public API, or configurations too casually. Don't make incompatible changes without good reasons!
 * Run `composer cs` to check the project for coding style issues and run `composer cbf` to fix them automatically where possible. These scripts use [`PHP_CodeSniffer`](https://github.com/squizlabs/PHP_CodeSniffer) in background.