client: Move client to client/v2 as a module.

We make v2 client code a module go.etcd.io/etcd/client/v2.

Pretty mechanical change that can be summarized as:

  mkdir client/v2
  cd client/v2 && git mod init go.etcd.io/etcd/client/v2

  git mv client/*.go client/v2/
  find -name '*.go' | xargs sed -i --follow-symlinks 's|/v3/client["]|/client/v2\"|g'

  + fixing changelog, bom, go.mod etc.

bill-of-materials.json

@@ -350,6 +350,15 @@
 			}
 		]
 	},
+	{
+		"project": "go.etcd.io/etcd/client/v2",
+		"licenses": [
+			{
+				"type": "Apache License 2.0",
+				"confidence": 1
+			}
+		]
+	},
 	{
 		"project": "go.etcd.io/etcd/pkg/v3",
 		"licenses": [