Extend "database is wrong version" with info about "gvmd --migrate"

[cfi-gb]

There are the following to messages:

https://github.com/greenbone/gvmd/blob/v20.8.0/src/gvmd.c#L2648

https://github.com/greenbone/gvmd/blob/v20.8.0/src/manage_sql.c (Line 934)

I don't know the details on both message and for which cases these are showing up but it could help for issues like #1270 to include some info on gvmd --migrate and other special cases (e.g. if a downgrade is tried) in that message.

[bjoernricks]

The message should be improved that it states gvmd --migrate should be run.