SDL_ttf: fix version string

While upstream has added release notes in their repo [1], no `2.0.11.1` version has been tagged.
According to the repology guidelines [2] that we mostly follow,
versioning of snapshots should be relative to the last tagged upstream release, which would be `2.0.11`.
The `2.0.11.1-unstable...` version string currently crashes the update script,
which is irritating when checking for all updates as a maintainer.

By the off-chance upstream does end up tagging a `2.0.11.1` later,
our version string would also incorrectly represent what we actually have.
Changelogs in git without a tag are still subject to change,
though unlikely in the case of SDL_ttf.

Whether we do or do not have the `.1` probably does not matter too much.

[1] https://github.com/libsdl-org/SDL_ttf/commit/e31d11a692e5b55e8e624ad766e4e44d655422c8
[2] https://repology.org/docs/requirements

(cherry picked from commit 1d5023fc)