MDL-65292 style: Fix all function declarations white space

This has been generated running the following Sniffs, all
them part of the Moodle's CodeSniffer standard:
- PSR12.Functions.ReturnTypeDeclaration
- PSR12.Functions.NullableTypeDeclaration
- moodle.Methods.MethodDeclarationSpacing
- Squiz.Whitespace.ScopeKeywordSpacing

All them are, exclusively, about correct spacing, so the changes
are, all them, only white space changes.

Only exceptions to the above are 3 changes what were setting the
return type in a new line, and, when that happens, the closing
parenthesis (bracket) has to go to the same line than the colon.

lib/classes/session/manager.php

@@ -1454,7 +1454,7 @@ class manager {
      * @param array $current
      * @return array
      */
-    private static function array_session_diff(array $previous, array $current) : array {
+    private static function array_session_diff(array $previous, array $current): array {
         // To use array_udiff_uassoc, the first array must have the most keys; this ensures every key is checked.
         // To do this, we first need to sort them by the length of their keys.
         $arrays = [$current, $previous];