Change vale rules to suggestions

What does this MR do?

Per #382360 (closed), changing a number of Vale rules from Warning to Suggestion so they do not display in the MR diff. These rules can be hard to fix directly in an MR using inline suggestions.

I also updated the messages in Spelling.yml and Uppercase.yml to instruct folks to ask a TW to add words to the exceptions list, instead of trying to do it by themselves.

Related issues

#382360 (closed)

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