Drupal 9 compatibility

Thanks for your update. Adding an interdiff file is appreciated for patch reviewing.

1. +++ b/custom_markup_block.info.yml
   @@ -1,6 +1,6 @@
   -core: 8.x
   

   Removing this statement makes the module incompatible with Drupal < 8.7.7, it should be present by now.

2. +++ b/composer.json
   @@ -11,5 +11,8 @@
   +  "require": {
   +    "drupal/core": "^8 || ^9"
   +    }
   

   It seems that adding default Drupal core itself to the module composer requirements is not mandatory. The module developer guide about creating a composer.json file does not (currently) mention it, but many of the most installed modules has no such dependency declaration, unless the module is not compatible with some specific core versions.

   For instance, the composer.json file of the webform module has no drupal/core listed in the requirement section, but it is present in the package information ( composer show -a drupal/webform ).

   There are some indentation errors.

3. +++ b/tests/src/Functional/CustomMarkupBlockTestBase.php
   @@ -10,6 +10,14 @@ use Drupal\Tests\block\Functional\BlockTestBase;
   +  /**
   +
   +   */
   

   Some garbage seems to be accidentally added here.

4. +++ b/custom_markup_block.info.yml
   @@ -1,6 +1,6 @@
   -version: 8.x-1.0
   

   Remove the version indication is right, it is commented out by the Drupal package system, but should be addressed in a different issue.

5. +++ b/tests/src/Functional/CustomMarkupBlockTestBase.php
   @@ -10,6 +10,14 @@ use Drupal\Tests\block\Functional\BlockTestBase;
   +  protected $defaultTheme = 'classy';
   

   According to this CR, the recommended default theme is Stark and should be used when possible.

   The install configuration of the block_test modules depends on the classy theme in Drupal < 9.0, so its value should be initialized during the setup depending on the Drupal core version.

Default theme is now dynamically set based on the core main version.

Wrong patch syntax.