Suppress PHP 8.1 warnings "Return type mixed is not covariant with tentative return type mixed of method Iterator::key()"

Summary:
PHP 8.1 requires classes which implement `Iterator` to have all `Iterator` methods declared with appropriate return types or annotated with `#[\ReturnTypeWillChange]`. In my (limited) understanding we cannot declare return types while remaining backward-compatible, thus annotate.

Closes T15835

Test Plan: Run PHPStan static code analyzer, see no warnings anymore.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15835

Differential Revision: https://we.phorge.it/D25711

src/applications/fact/extract/PhabricatorFactUpdateIterator.php

@@ -33,6 +33,7 @@ final class PhabricatorFactUpdateIterator extends PhutilBufferedIterator {
     }
   }
 
+  #[\ReturnTypeWillChange]
   public function key() {
     return $this->getCursorFromObject($this->current());
   }

src/applications/harbormaster/storage/build/HarbormasterBuildLogChunkIterator.php

@@ -18,6 +18,7 @@ final class HarbormasterBuildLogChunkIterator
     $this->cursor = $this->min;
   }
 
+  #[\ReturnTypeWillChange]
   public function key() {
     return $this->current()->getID();
   }

src/infrastructure/storage/lisk/LiskMigrationIterator.php

@@ -26,6 +26,7 @@ final class LiskMigrationIterator extends PhutilBufferedIterator {
     $this->cursor = 0;
   }
 
+  #[\ReturnTypeWillChange]
   public function key() {
     return $this->current()->getID();
   }

src/infrastructure/storage/lisk/LiskRawMigrationIterator.php

@@ -16,6 +16,7 @@ final class LiskRawMigrationIterator extends PhutilBufferedIterator {
     $this->cursor = 0;
   }
 
+  #[\ReturnTypeWillChange]
   public function key() {
     return idx($this->current(), $this->column);
   }

src/infrastructure/storage/lisk/PhabricatorQueryIterator.php

@@ -18,6 +18,7 @@ final class PhabricatorQueryIterator extends PhutilBufferedIterator {
     $this->pager = $pager;
   }
 
+  #[\ReturnTypeWillChange]
   public function key() {
     return $this->current()->getID();
   }