New PMIx v2.x release for OMPI v3.1.x

[jsquyres]

@rhc54 says there have been a few bug fixes that have accumulated in the PMIx v2.x series such that we can get a new PMIx release/drop fro the OMPI v3.1.x branch.

[jsquyres]

Putting "blocker" on here because the PMIx refresh is a gating factor.

[jsquyres]

#6778 was merged, which brought in PMIx v2.2.3rc1. If that clears MTT successfully, there will be a follow-on PR to bring it up to PMIx v2.2.3 (final).

[rhc54]

Done