In practice, there isn't a simple query that will do that: SQL string handling is basic at best. '%' matches any number of characters (including zero) and while there is a "match any one character": underscore (which I can't type as it triggers Markdown to "italice mode" instead of displaying it - it's a site bug) it will always match one and only one character.
So
LIKE '2._'
would solve the immediate problem and match "2.1" and "2.2" without matching "2.1.1" - but it won't find "2.11" either.
I'd strongly suggest that you need to do this in your presentation software, or change your DB design to make life easier for SQL to process this stuff.
You can probably do it - start by looking at this:
Converting comma separated data in a column to rows for selection[
^] which separates CSV style data and woudl be easy to mod to your period separated data - but then you need to work out how to use that to "limit" your selections. Not an easy job, and seriously inefficient on big tables!
I'd spend some time looking at the data and seeing if there is a better way to store it which makes this kind of query easier, now and in the future.