remove mixed decl deprecation silence, add deprecation silence for if-function

[OyvindLGjesdal]

GitHub issue resolved #3655

Pull request Description:

It looks like the new if-function warnings are well known for bootstrap and the temporary fix is silencing them.

twbs/bootstrap#41915

I took a look in Jena UI, and think none of the deprecation warnings for if-function are native to Jena.

Also removed the obselete silence for mixed-decls https://sass-lang.com/documentation/breaking-changes/mixed-decls/

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• Tests are included.
• Documentation change and updates are provided for the Apache Jena website
• Commits have been squashed to remove intermediate development commit messages.
• Key commit messages start with the issue number (GH-xxxx)

By submitting this pull request, I acknowledge that I am making a contribution to the Apache Software Foundation under the terms and conditions of the Contributor's Agreement.

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See the Apache Jena "Contributing" guide.

[afs]

Thank you!