[Process] Deprecate using values that are not string for Process::setStdin and ProcessBuilder::setInput
feature #10930 [Process] Deprecate using values that are not string for Process::setStdin and ProcessBuilder::setInput (romainneutron)

This PR was merged into the 2.4-dev branch.

Discussion
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[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.

Commits
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9887b83 [Process] Deprecate using values that are not string for Process::setStdin and ProcessBuilder::setInput
[Debug] preserve modified error level