[Process] Deprecate using values that are not string for Process::setStdin and ProcessBuilder::setInput

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT

This deprecates passing a `Process` input any value that is not a strict string. This needs #10929 to be merged.
I don't know if the use of `trigger_error` is correct or should be removed.