Replace all CompatHelpers CI across TuringLang with dependabot.

[yebai]

One advantage of Dependabot is that it generates fewer noisy PRs. CompatHelper currently opens one PR per environment, which also makes testing awkward: for example, testing a dependency update properly may require merging both the project-environment and test-environment PRs.

See, eg,

• https://github.com/SciML/ComponentArrays.jl/blob/main/.github/dependabot.yml
• Bump the all-julia-packages group across 1 directory with 11 updates SciML/ComponentArrays.jl#368