Lower mbstring dep, remove it for Yaml and CssSelector components
minor #10890 Lower mbstring dep, remove it for Yaml and CssSelector components (nicolas-grekas)

This PR was merged into the 2.3 branch.

Discussion
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Lower mbstring dep, remove it for Yaml and CssSelector components

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | none

Commits
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a4b805d Lower mbstring dep, remove it for Yaml and CssSelector components