Bump version numbers to 3.15

[ffaf1]

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog. N/A, changelog will we the release itself.
• The documentation has been updated, if necessary.
• n/a (just version change) Manual QA notes have been included.
• n/a Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)

--

This updates bootstrap files too and (important!) index-state.

[fgaz]

hackage-security pr: haskell/hackage-security#319

[ffaf1]

This PR incorporates a necessary bump of index-state. From the wiki:

Open a PR bumping the index state in https://github.com/haskell/cabal/blob/master/cabal.release.project (and the relevant release branch) and any other files that fix the index state and ask for volunteers to handle the fallout, if any.

I await the fallout. What kind of fallout are we expecting? It would be a useful bit of info to add to the wiki.

[ffaf1]

Twin PR for 3.14 is #10310

[ulysses4ever]

👍

[geekosaur]

One I'm particularly worried about is #10284 coming back when that bound is bumped.