* SDL2: checkout tag `release-2.30.1`
* SDL2_image: checkout tag `release-2.8.2`
* SDL2_ttf: checkout tag `release-2.22.0`
* glm: checkout tag `1.0.1`
* solarus: checkout branch `dev`
* openal-soft: checkout tag `1.23.1`

Also reverted to using default branches in `.gitmodules` file.