Rollup merge of #140904 - ehuss:future-incompat-template, r=Mark-Simulacrum

Add an issue template for future-incompatible lints

This adds a GitHub issue template for future-incompatible lints. Most of the existing tracking issues have been using different formats with different information, and I think it would be helpful to make them a little more consistent and to ensure that sufficient information is provided.

Some comments on my choices:
* Added a dedicated section to describe *why* the change is being made. Many existing issues already have this, so let's standardize on it.
* Have a section with a very clear example. Almost all of the existing issues have this in one form or another.
* Added a "Recommendations" section, since this is something I see missing in several of the existing issues, and this is really important information IMHO.
* I reworded the "When" section. The existing template mentioned that these get reviewed every 6 weeks which my understanding is not true. That's also not very helpful information to the user, since it doesn't really answer the question. I'm not sure this section will actually be useful since I suspect most of the time we don't know when it will change (there have been a few exceptions).
* Clearly show the expected progression steps. Several issues already have this.
* Added implementation history, which is useful for linking PRs. (IDK, this could get merged with "Steps".)