[java-shared-config] flatten-maven-plugin v1.6.0 is causing downstream checks failures

upgrading flatten-maven-plugin from v1.3.0 to v1.6.0 is causing downstream checks failures for java-spanner and java-spanner-jdbc in sdk-platform-java
see https://github.com/googleapis/sdk-platform-java/pull/2512, https://github.com/googleapis/sdk-platform-java/pull/2571#issuecomment-1997618589

```
Error: com.google.cloud.spanner.executor.v1.SpannerExecutorProxyClientTest.executeActionAsyncTest -- Time elapsed: 0.039 s <<< ERROR!
java.lang.VerifyError:
Bad type on operand stack
Exception Details:
Location:
com/google/spanner/executor/v1/SpannerActionOutcome$Builder.setCommitTime(Lcom/google/protobuf/Timestamp$Builder;)Lcom/google/spanner/executor/v1/SpannerActionOutcome$Builder; @42: invokevirtual
Reason:
Type 'com/google/protobuf/Timestamp' (current frame, stack[1]) is not assignable to 'com/google/protobuf/AbstractMessage'
Current Frame:
bci: @42
flags: { }
locals: { 'com/google/spanner/executor/v1/SpannerActionOutcome$Builder', 'com/google/protobuf/Timestamp$Builder', top, '[Z' }
stack: { 'com/google/protobuf/SingleFieldBuilderV3', 'com/google/protobuf/Timestamp' }
Bytecode:
0000000: b803 d04e 2ab4 0075 c700 1a2d 1101 1d04
0000010: 542a 2bb6 0211 b500 712d 1101 1e04 54a7
0000020: 0015 2ab4 0075 2bb6 0211 b601 ed57 2d11
0000030: 011f 0454 2a59 b400 6005 80b5 0060 2d11
0000040: 0120 0454 2ab6 016c 2a2d 1101 2104 54b0
0000050:
Stackmap Table:
append_frame(@34,Top,Object[#978])
same_frame(@52)

at com.google.spanner.executor.v1.SpannerActionOutcome.toBuilder(SpannerActionOutcome.java:893)
at com.google.spanner.executor.v1.SpannerActionOutcome.newBuilder(SpannerActionOutcome.java:884)
at com.google.cloud.spanner.executor.v1.SpannerExecutorProxyClientTest.executeActionAsyncTest(SpannerExecutorProxyClientTest.java:89)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:568)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
at org.apache.maven.surefire.junit4.JUnit4Provider.execute(JUnit4Provider.java:316)
at org.apache.maven.surefire.junit4.JUnit4Provider.executeWithRerun(JUnit4Provider.java:240)
at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:214)
at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:155)
at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:385)
at org.apache.maven.surefire.booter.ForkedBooter.execute(ForkedBooter.java:162)
at org.apache.maven.surefire.booter.ForkedBooter.run(ForkedBooter.java:507)
at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:495)
```

added temp config to renovate bot in https://github.com/googleapis/java-shared-config/pull/786 ignoring this deps.

Need to investigate and remove above ignore when able to upgrade.