rebase: bump google.golang.org/grpc from 1.65.0 to 1.66.0

Bumps [google.golang.org/grpc](https://github.com/grpc/grpc-go) from 1.65.0 to 1.66.0.
- [Release notes](https://github.com/grpc/grpc-go/releases)
- [Commits](https://github.com/grpc/grpc-go/compare/v1.65.0...v1.66.0)

---
updated-dependencies:
- dependency-name: google.golang.org/grpc
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

vendor/google.golang.org/grpc/internal/envconfig/envconfig.go

@@ -46,6 +46,10 @@ var (
 	// by setting the environment variable "GRPC_ENFORCE_ALPN_ENABLED" to "true"
 	// or "false".
 	EnforceALPNEnabled = boolFromEnv("GRPC_ENFORCE_ALPN_ENABLED", false)
+	// XDSFallbackSupport is the env variable that controls whether support for
+	// xDS fallback is turned on. If this is unset or is false, only the first
+	// xDS server in the list of server configs will be used.
+	XDSFallbackSupport = boolFromEnv("GRPC_EXPERIMENTAL_XDS_FALLBACK", false)
 )
 
 func boolFromEnv(envVar string, def bool) bool {