Merge branch 'cherry-pick-a7b56918-21.2' into '21.2'

Sweeping !47656 from master to 21.2.
AsgServices: Use std::nullptr_t for gcc11

See merge request !47688

Control/AthToolSupport/AsgServices/AsgServices/IAsgService.h

@@ -21,7 +21,7 @@
 /// followed by a semicolon, so we need a statement that requires to
 /// be followed by a semi-colon.
 #define DeclareInterfaceID(iface, major, minor) \
-  static constexpr nullptr_t interfaceID = nullptr
+  static constexpr std::nullptr_t interfaceID = nullptr
 #endif
 
 namespace asg