Better version dropdown support for semantic versioning [#2476565]

Problem/Motivation

There are currently two drawbacks with the version dropdown for core in terms of 8.0.x vs. 8.1.x:

- 8.1.x was branched just to allow it to show up in the dropdown, it's out of date and this often confuses people. If we regularly merged into 8.1.x that also might indicated it's open for development, which would also be confusing.

- right now, all core issues are against 8.0.x or 8.1.x. An issue assigned to 8.1.x now means "could get into a minor release of 8.1.x (as opposed to waiting until 9.x), but won't get into 8.0.x".

Given those issues apply to minor versions, once 8.1.0 is tagged (or a beta/rc before that), every issue against 8.1.x at that point will have to move to 8.2.x (if it's a 'minor version target'). Conversely once 8.1.0 is tagged, all the 8.0.x issues will need to move to 8.1.x (since they'll be eligible for patch-level releases).

Proposed resolution

All of this makes me think it would be useful for the drop-down to support the concept of 'upcoming minor version' (and probably 'upcoming major version') without needing to pin to a specific version every six months.