man: Update minimum required OpenGL version

[ventureoo]

Summary

Since old backends have been dropped, I think it should be clarified that the current minimum required version of OpenGL is 3.3 (for EGL AFAIK, required version is 4.2, but man page doesn't mention the presence of EGL backend for some reason).

Checklist

• [x] Enable "Allow edits from maintainers". (So we can make necessary changes).

• [ ] Add changelog entries to commit messages when appropriate. These will be included in release announcements.

  Format the changelog entries like this:

  Changelog: BugFix: Stop picom from summoning the Great Old Ones.

  The entry starts with Changelog: , followed by a category name (BugFix: in this case), then a short description of the changes in the commit.

  How to format changelog entries