revert: undo parent POM restructuring and property consolidation

Undoes the previous property centralization and parent POM restructuring in response to PR review feedback:
- Restores skipUnitTests and checkstyle.header.file properties to their original files: google-cloud-pom-parent/pom.xml and gapic-generator-java-pom-parent/pom.xml.
- Restores gapic-generator-java-pom-parent/pom.xml to inherit directly from google-cloud-shared-config.
- Reverts all changes to sdk-platform-java-config/pom.xml.
- Retains Error Prone version updates (2.48.0) and redundant compiler flag cleanups.

BUG=b/481669998
TAG=agy
CONV=7b193e4d-38f8-450c-8475-0b9ae0d04507

Author: lqiu96

google-cloud-pom-parent/pom.xml

@@ -20,6 +20,9 @@
   </parent>
 
   <properties>
+    <skipUnitTests>false</skipUnitTests>
+    <checkstyle.header.file>java.header</checkstyle.header.file>
+    <errorprone.version>2.48.0</errorprone.version>
     <!-- By default, no special internal javac flag is needed on JDK 22+.
          For JDK 21 specifically, profile errorprone-jdk21 activates the required flag. -->
     <errorprone.javac.args>-g</errorprone.javac.args>

sdk-platform-java/gapic-generator-java-pom-parent/pom.xml

@@ -14,12 +14,13 @@
   </description>
   <parent>
     <groupId>com.google.cloud</groupId>
-    <artifactId>sdk-platform-java-config</artifactId>
-    <version>3.63.0</version><!-- {x-version-update:google-cloud-shared-dependencies:current} -->
-    <relativePath>../sdk-platform-java-config</relativePath>
+    <artifactId>google-cloud-shared-config</artifactId>
+    <version>1.17.0</version>
   </parent>
 
   <properties>
+    <skipUnitTests>false</skipUnitTests>
+    <checkstyle.header.file>java.header</checkstyle.header.file>
     <maven.compiler.release>8</maven.compiler.release>
 
     <!-- External dependencies, especially gRPC and Protobuf version, should be
@@ -31,6 +32,7 @@
     <guava.version>33.5.0-jre</guava.version>
     <protobuf.version>4.33.2</protobuf.version>
     <opentelemetry.version>1.62.0</opentelemetry.version>
+    <errorprone.version>2.48.0</errorprone.version>
     <j2objc-annotations.version>3.1</j2objc-annotations.version>
     <threetenbp.version>1.7.0</threetenbp.version>
     <junit.version>5.11.4</junit.version>

sdk-platform-java/sdk-platform-java-config/pom.xml

@@ -21,8 +21,5 @@
         <google-cloud-shared-dependencies.version>3.63.0</google-cloud-shared-dependencies.version> <!-- {x-version-update:google-cloud-shared-dependencies:current} -->
         <!--  skipping clirr check for protobuf 4.x upgrade for downstream libraries -->
         <clirr.skip>true</clirr.skip>
-        <skipUnitTests>false</skipUnitTests>
-        <checkstyle.header.file>java.header</checkstyle.header.file>
-        <errorprone.version>2.48.0</errorprone.version>
     </properties>
 </project>