Add Vale to .tool-versions file

What does this MR do?

Like we're now doing in the gitlab project: https://gitlab.com/gitlab-org/gitlab/-/blob/a891058bb057e7b40513d3c081782ac8f507567b/.tool-versions, we can manage the version of Vale people use. Sometimes the latest version is too far ahead of our pipeline configuration, where we do pin the version: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/333454cedc044729748190a904b48c98e2089178/.gitlab-ci.yml#L25.

Previously, we relied on people brew installing Vale or similar.

CC @marcel.amirault @axil

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