Although we don't typically do configuration-level string wrangling
directly in Terraform, we delegate to several other upstream libraries
that do. These upgrades all switch to newer versions that support the
latest definitions from Unicode 13, primarily affecting operations such
as converting strings to upper/lowercase or splitting strings into
component characters (substr, reverse, etc).

The tests for the upstream libraries didn't show any regressions from
these updates, so the Unicode 13 changes seem to be backward-compatible
additions rather than significant breaking changes.

(Our go.mod file had also become non-canonical in some ways, and the Go
toolchain fixed that as part of this work, causing a few extra style-only
diffs here that shouldn't cause any change in behavior.)