README.md: Revert reference to protobuf 3.23.4 (#10430)

The README describes the released version of gRPC. But the most recent
release doesn't use protobuf 3.23.4. Telling people to use protoc 3.23.4
is asking for breakages because they will be running with protobuf-java
3.22.3, which is a downgrade.

The bump to 3.23.4 was incorrectly done in #10359.

README.md

@@ -129,7 +129,7 @@ For protobuf-based codegen integrated with the Maven build system, you can use
       <artifactId>protobuf-maven-plugin</artifactId>
       <version>0.6.1</version>
       <configuration>
-        <protocArtifact>com.google.protobuf:protoc:3.23.4:exe:${os.detected.classifier}</protocArtifact>
+        <protocArtifact>com.google.protobuf:protoc:3.22.3:exe:${os.detected.classifier}</protocArtifact>
         <pluginId>grpc-java</pluginId>
         <pluginArtifact>io.grpc:protoc-gen-grpc-java:1.57.1:exe:${os.detected.classifier}</pluginArtifact>
       </configuration>
@@ -157,7 +157,7 @@ plugins {
 
 protobuf {
   protoc {
-    artifact = "com.google.protobuf:protoc:3.23.4"
+    artifact = "com.google.protobuf:protoc:3.22.3"
   }
   plugins {
     grpc {
@@ -190,7 +190,7 @@ plugins {
 
 protobuf {
   protoc {
-    artifact = "com.google.protobuf:protoc:3.23.4"
+    artifact = "com.google.protobuf:protoc:3.22.3"
   }
   plugins {
     grpc {