Gun Damage Booster v7.5

* Added initial changes for v7.5

- Lowered the limit of the damage boost settings from `999999` to `99999`.
- Added the `sm_gdb_damage` command. (Requested by `weffer`.)
- Added the `gdb_friendlyfire` convar.
- Added the `gdb_launcher` convar. (Requested by `Sunyata`.)
- Damage boosts are now applied to Witches. (Requested by `Sunyata`.)
- Made the `gdb_pluginversion` convar unchangeable outside of the source code.

* Added more changes for v7.5

- Fixed `OnMapEnd` not triggering due to a typo.
- Added the `sm_boost` command. (Requested by `gamer_kanelita` and `Lagg`.)
- Renamed the `sm_gdb_damage` command to `sm_damage`.
- Added notification flags to each convar.
- Added the `gdb_global` convar.

* Updated README.md

* Updated README.md