Bugfix: grpc python code generation location and file suffix

[fliiiix]

Follow up from #8356.

This addresses the regression for the location where the grpc file is generated and changes the default suffix back to _fb again instead of .fb.

(See commit messages for more details).

[fliiiix]

Can this be merged @dbaileychess or does it need something else?

[fliiiix]

Any update if this can be merged? @dbaileychess @anton-bobukh

[fliiiix]

Rebased to master any chance this can be merged @dbaileychess @anton-bobukh as this still breaks imports in python

[fliiiix]

rebased again

[fliiiix]

@aardappel or @dbaileychess any chance this can also be merged to fix the regression after this was merged now https://github.com/google/flatbuffers/pull/8500

[aardappel]

CI issues seem unrelated

[aardappel]

Thanks :)

[fliiiix]

Thank you :sparkles: good to see that flatbuffers is still maintained