downloads - Old Stable is confusing - remove

Old Stable has been used for releases for a while. We don't however give it a formal meaning.

We are using this for 10.2+ -> 10.5 versions currently.

A user could easily be confused by this an arrive at any of the following incorrect conclusions:

• There are no more updates on these version.
• There are only security updates on these versions (Like Debian Old Stable).
• The old stable versions are less supported by MariaDB
• The old stable versions are more stable than new stable (probably actually true, but not intentionally so).

Given this ambiguity can we remove the use of the term.