Create deprecations template

What does this MR do?

This MR is to address point 2 in this follow up comment regarding improving the deprecations process.

I think there should be a deprecation issue template that has a checklist to do these things, that PMs/EMs can use when creating a new deprecation (we can use the feature issue template and simplify it). This template can then be leveraged to have the appropriate labels/section headers for @mjwood exploration of using the release post item generator to auto generate the MRs as needed...