MDEV-8978 Specify GPL version in RPM metadata

The License field in the MariaDB RPM packages is GPL. This does not tell
the version of GPL and might confuse tools and users that rely on this
field.

Best practice in the RPM world is to use "GPLv2" for the GPL 2.0
license.

The commit switches the license field of the RPM packages to GPLv2.

cmake/cpack_rpm.cmake

@@ -31,7 +31,7 @@ SET(CPACK_RPM_PACKAGE_NAME "MariaDB")
 SET(CPACK_PACKAGE_FILE_NAME "${CPACK_RPM_PACKAGE_NAME}-${VERSION}-${RPM}-${CMAKE_SYSTEM_PROCESSOR}")
 
 SET(CPACK_RPM_PACKAGE_RELEASE "1%{?dist}")
-SET(CPACK_RPM_PACKAGE_LICENSE "GPL")
+SET(CPACK_RPM_PACKAGE_LICENSE "GPLv2")
 SET(CPACK_RPM_PACKAGE_RELOCATABLE FALSE)
 SET(CPACK_RPM_PACKAGE_GROUP "Applications/Databases")
 SET(CPACK_RPM_PACKAGE_URL "http://mariadb.org")