Partially revert 1414641 to resurrect 2.1 upgrade code

[dlmarion]

No description provided.

[ctubbsii]

I was in the middle of reviewing this when it got merged. It mostly looks okay, but there are still some differences in the changed files after things got merged that I'm trying to ensure are okay.

git diff main elasticity -- $(git log --name-status fc2a2e7dd32ac52da3e861c2f05b126b3b07d7b0 -n1 --oneline | tail -n+2 | cut -c2-)

[dlmarion]

I think that the differences are due to the upgrader being disabled in elasticity right now. Getting the upgrade working for 4.0 is a TODO.

[ctubbsii]

Yeah, that's definitely the main diff, and that's fine. But, I think there's some stuff that should be sync'd with main but aren't. I'll do a follow-on PR for those things I think should be fixed. They're pretty trivial.

[ctubbsii]

See #4003 for the fixes. The other remaining difference is the use of Ample instead of KeyExtent.isMeta in the UpgradeCoordinator code. However, that change wasn't as the result of incomplete conflict resolution, like what #4003 fixes. Rather, that difference is due some changes as part of #3904 that was only applied to the elasticity branch. Those particular changes may be able to be backported to main, as they may not be elasticity-specific. But, it's probably not critical to do so.