upcxx: Update the UPC++ package to 2021.9.0 (#26996)

* upcxx: Update the UPC++ package to 2021.9.0

* Add the new release, and a missing older one.

* Remove the spack package cruft for supporting the obsolete build system that
  was present in older versions that are no longer supported.

* General cleanups.

Support for library versions older than 2020.3.0 is officially retired,
for two reasons:

1. Releases prior to 2020.3.0 had a required dependency on Python 2,
   which is [officially EOL](https://www.python.org/doc/sunset-python-2/)
   as of Jan 1 2020, and is no longer considered secure.
2. (Most importantly) The UPC++ development team is unable/unwilling to
   support releases more than two years old.  UPC++ provides robust
   backwards-compatibility to earlier releases of UPC++ v1.0, with very
   rare well-documented/well-motivated exceptions.  Users are strongly
   encouraged to update to a current version of UPC++.

NOTE: Most of the lines changed in this commit are simply re-indentation,
and thus might be best reviewed in a diff that ignores whitespace.

* upcxx: Detect Cray XC more explicitly

This change is necessary to prevent false matches occuring on new Cray Shasta
systems, which do not use the aries network but were incorrectly being treated
as a Cray XC + aries platform.

UPC++ has not yet deployed official native support for Cray Shasta, but this
change is sufficient to allow building the portable backends there.

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