Make Java defined functions nullary when we override them

[hamzaremmal]

To preserve the same source compatibility, we mark toString and hashCode as methods with empty parameter lists in all the inherited files from Scala 2. Note that I checked for other java defined methods that would qualify for this change but I couldn't find any (reviewer should double check too since we cannot fix this after 3.8.0).

Closes #19616
Supersedes #24452

[som-snytt]

If Scala 3 is not going to add parens to the overriding definition, surely it should warn. Otherwise it's a gotcha forever.

I'll contribute a warning if it is deemed useful.

[hamzaremmal]

No need for a warning, this is only to not break source compatibility and already existing macros. Scala 2 already added those parentheses for Java defined methods. This is just porting those changes only in the Scala 2 inherited files. You see that Tuple.scala didn't change this.

@sjrd I just saw that I had some unsaved changes locally that had to do with this PR, I'll include them and go over the list a second time.

@som-snytt thank you for the help!