[TASK] Declare non static methods as static

Change-Id: Ifa199fd6b500f4f161b49a406aa2678abdaa785b
Resolves: #45509
Releases: 6.1
Reviewed-on: https://review.typo3.org/18301
Reviewed-by: Dmitry Dulepov
Reviewed-by: Oliver Klee
Reviewed-by: Simon Schaufelberger
Tested-by: Simon Schaufelberger
Reviewed-by: Soren Malling
Tested-by: Soren Malling
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn