As far as I know, I think the 4.13.1 code is fine to use now. The template should be updated. Dependabot doesn't know how to update this version string, so it has to be done manually.
There doesn't seem to be any Cpp code diffs under github.com/antlr/runtime/Cpp for 4.13.1. git diff 7ed420f 0ae86c2 -- .. The tool itself was changed slightly, so it is probably good to just make sure it all works for 4.13.1.
Note, there is only one grammar that requires 4.13.1 and that is python/python3. The desc.xml lists that it should only be tested for 4.13.1 but it seems it's tested anyway--using 4.13.0. I probably don't do any integrity checking that the version specified in the desc.xml is the same as that in the templates. That's a bug.
Everything that was working still seems to be working (I'm still running the testing for Cpp on the whole repo).
As far as I know, I think the 4.13.1 code is fine to use now. The template should be updated. Dependabot doesn't know how to update this version string, so it has to be done manually.