This reverts part of 36cda3b3116acba3b895daf68689f8af01b62392
and replaces the use of `OB_MODE_PAINT_GREASE_PENCIL`
`OB_MODE_PAINT_GPENCIL_LEGACY` flag instead.
The `OB_MODE_PAINT_GREASE_PENCIL` is removed.
The `GREASE_PENCIL_OT_draw_mode_toggle` operator is removed
and the `GPENCIL_OT_paintmode_toggle` operator is adapted to
work with GPv3.
Pull Request: https://projects.blender.org/blender/blender/pulls/121027