This PR replaces various class name strings by the ::class constants. The remaining occurrences of such strings can be replaced in other PRs.

we refused such changes because it complicates merges from 2.8 to master for no practical benefit...

See #16648 that has just been closed

Ah ok.

I thought about adding a NAME constant to all form types in 2.3 that have the shorthand name in 2.3-2.7 and the FQCN in 2.8. In 3.0 we can then just change the constant to const NAME = self::class;.

Then we could use the NAME constant in all code and documentation and in all branches. I guess that would be easier to maintain and more readable.
But it also involves alot of merges.