groupcache: add go.mod, update for Go 1.24 (#175)

Add a go.mod file

As of Go 1.24 rand.Seed is a no-op. Change TestPeers to use an explicit rand.Rand instead.

• go.mod[Added - diff]
• go.sum[Added - diff]
• groupcache.go[diff]
• groupcache_test.go[diff]