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 <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: thepudds

src/internal/buildcfg/exp.go

@@ -83,7 +83,6 @@ func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
 		RegabiArgs:            regabiSupported,
 		Dwarf5:                dwarf5Supported,
 		RandomizedHeapBase64:  true,
-		RuntimeFree:           true,
 		SizeSpecializedMalloc: true,
 		GreenTeaGC:            true,
 	}

src/internal/goexperiment/exp_runtimefree_off.go

@@ -113,8 +113,8 @@ type Flags struct {
 	// platforms.
 	RandomizedHeapBase64 bool
 
-	// RuntimeFree enables the runtime to free and reuse memory more eagerly in some circumstances with compiler help.
-	RuntimeFree bool
+	// RuntimeFreegc enables the runtime to free and reuse memory more eagerly in some circumstances with compiler help.
+	RuntimeFreegc bool
 
 	// SizeSpecializedMalloc enables malloc implementations that are specialized per size class.
 	SizeSpecializedMalloc bool