Submodules, by definition, always reference particular SHA1 in the subproject. That SHA1 isn’t expressed in the .gitmodules file, but is instead expressed as the entry in the tree object that contains the submodule. The way you set this in git is by cding into the submodule, checking out the SHA1 you want, then cding back to the parent repo and committing your change, which will show up just like a changed file.
So in your case what you can do is
cd vendor/plugins/ssl_requirement
git checkout bc96ad96407a72a60e0542cf3b0cecc6ff9e278e
cd ..
git add ssl_requirement
# commit whenever you're ready