runtime: GODEBUG=tracebacklabels=1 for 1.27+

https://go.dev/cl/742580 updated the goroutine label format in
tracebacks to only quote the values under some circumstances, and use a
more broadly acceptable format. Flip the GODEBUG=tracebacklabels default
for go 1.27+ modules.

Notably, tracebacklabels is marked as Opaque to avoid some diciness in
maintaining a usage metric in signal handlers, etc. (where mutex
acquisition may be problematic)

Also, update the go 1.27 release notes to call out the new feature.

Updates #76349

Change-Id: Iedd3f6b17d83e99da20365b952a565022d20ff13
Reviewed-on: https://go-review.googlesource.com/c/go/+/751580
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>

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