Fix major version calculation for two digit versions

Support version 20.04 and following.

Author: bjoernricks

gvm/protocols/gmp.py

@@ -98,7 +98,7 @@ def determine_supported_gmp(self) -> SupportedGmpVersion:
             corresponding Gmp class instance
         """
         version = self.determine_remote_gmp_version()
-        major_version = int(version[0])
+        major_version = int(version.split('.')[0])
         if major_version == 7:
             gmp_class = Gmpv7
         elif major_version == 8: