Use libs.versions directly for protobuf & grpc plugins

Since we have moved all the version management for Gradle to the TOML,
there is no need to extract `ext` `protobufVersion` & `grpcVersion` entries.

* Use `libs.versions.protobufVersion.get()` & `libs.versions.grpcVersion.get()`
directly in the `protobuf` Gradle plugin

This way main `build.gradle` becomes more cleaner and consice

Author: artembilan

build.gradle

@@ -34,9 +34,6 @@ ext {
     linkScmDevConnection = 'git@github.com:spring-projects/spring-integration.git'
 
     javaProjects = subprojects - project(':spring-integration-bom')
-
-	protobufVersion = libs.versions.protobufVersion.get()
-	grpcVersion = libs.versions.grpcVersion.get()
 }
 
 allprojects {
@@ -449,7 +446,7 @@ project('spring-integration-core') {
 
     protobuf {
         protoc {
-            artifact = "com.google.protobuf:protoc:$protobufVersion"
+            artifact = "com.google.protobuf:protoc:${libs.versions.protobufVersion.get()}"
         }
     }
 
@@ -566,11 +563,11 @@ project('spring-integration-grpc') {
 
 	protobuf {
 		protoc {
-			artifact = "com.google.protobuf:protoc:$protobufVersion"
+			artifact = "com.google.protobuf:protoc:${libs.versions.protobufVersion.get()}"
 		}
 		plugins {
 			grpc {
-				artifact = "io.grpc:protoc-gen-grpc-java:$grpcVersion"
+				artifact = "io.grpc:protoc-gen-grpc-java:${libs.versions.grpcVersion.get()}"
 			}
 		}
 		generateProtoTasks {