runtime: set GOEXPERIMENT=runtimefreegc to disabled by default

runtime: set GOEXPERIMENT=runtimefreegc to disabled by default

This CL is part of a set of CLs that attempt to reduce how much work the
GC must do. See the design in https://go.dev/design/74299-runtime-freegc

The plan has been for GOEXPERIMENT=runtimefreegc to be disabled
by default for Go 1.26, so here we disable it.

Also, we update the name of the GOEXPERIMENT to reflect the latest name.

Updates #74299

Change-Id: I94a34784700152e13ca93ef6711ee9b7f1769d9a
Reviewed-on: https://go-review.googlesource.com/c/go/+/720120
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>