loading: Clamp Julia syntax version from Manifest as well (#60275)
This is really a Pkg bug, where I forgot to implement the clamping when
generating the manifest (and will submit a PR), but since we clamp and
override when loading from a Project.toml, it seems reasonable to do the
same for Manifest.toml. We never want to use a syntax version less than
1.13, so cut off that path as well, without assuming that Pkg.jl gets it
right (too much manual editing of Manifest files happening with AI
assistants).
Fixes #60273
Written by Claude.