Fix backup for Colvars output files

Makes the backup convention for Colvars output files consistent with other GROMACS output files (i.e. multiple backups). Currently, only a single backup file with either the .old or .BAK suffix appended is being produced, which is a convention inherited from NAMD. See #5071 (closed)

The revised interface code appends _prev to the Colvars state file's name (because is usually written at the same time as a checkpoint), and uses make_backup() otherwise.

This change may be cherry-picked on top of main.

A patch version number is also introduced for the Colvars library, to distinguish it from the version originally bundled with GROMACS 2024.0. The Colvars patch number is currently set to 2, because the GROMACS 2024.2 is being targeted here. The patch version does not need to be cherry-picked onto main, because it will be eventually reset when new Colvars features are added.