[clang-tidy] Rename 'cert-dcl50-cpp' to 'modernize-avoid-variadic-functions' (#157737)
Chose `modernize` section because we already have
`modernize-avoid-bind`, `modernize-avoid-c-arrays` and this check sound
modernize-ish (but next place it could go is `misc` I guess).
Closes https://github.com/llvm/llvm-project/issues/157301.