Deprecate WebApp#mimeTypes

[basil]

In practice this map is always empty, since I checked Jenkins and CloudBees sources and nothing ever writes to the map. The only reader besides Stapler is https://github.com/jenkinsci/scm-api-plugin/blob/a24e62fa67977d1c381b922a1a745afb725771c7/src/main/java/jenkins/scm/api/SCMFile.java#L422. Altogether I can't see a good reason to retain this feature; the servlet container should always be the source of truth and this seems like yet another case of Stapler over-reaching in scope. Simpler to deprecate this feature and remove it eventually.

Testing done

mvn clean verify

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