Draft: Issue #3517435: Drop support for D8, D9, D10.0-10.3

Closes #3517435

I've put this in Draft because we would actually want to commit this to a 3.0.x branch, not the 2.x branch.

src/Controller/BlocksController.php

@@ -40,7 +40,7 @@ private function getEditableConfig() {
   /**
    * Function to export custom blocks.
    */
-  public function exportBlocks(array $form = NULL, FormStateInterface $form_state = NULL) {
+  public function exportBlocks(?array $form = NULL, ?FormStateInterface $form_state = NULL) {
     StructureSyncHelper::logMessage('Custom blocks export started');
 
     if (is_object($form_state) && $form_state->hasValue('export_block_list')) {
@@ -109,7 +109,7 @@ public function exportBlocks(array $form = NULL, FormStateInterface $form_state
    * an array with a key value pair for form with key 'style' and value 'full',
    * 'safe' or 'force' to apply that import style.
    */
-  public function importBlocks(array $form, FormStateInterface $form_state = NULL) {
+  public function importBlocks(array $form, ?FormStateInterface $form_state = NULL) {
     StructureSyncHelper::logMessage('Custom blocks import started');
 
     // Check if the import style has been defined in the form (state) and else