build(deps): maven-gpg-plugin 3.2.7 in the generator

[suztomo]

Let's use the latest maven-gpg-plugin in the generator. It seems that there's a concurrency problem in the old versions.

b/421886950

This version matches what we have in https://github.com/googleapis/java-shared-config/blob/main/native-image-shared-config/pom.xml#L190C17-L200C32.

I'll create another CL that modify the generated pom.xml files.

[suztomo]

Memo for the CI failure below. I remember some modules do not compile.

The following libraries cannot be compiled:
[INFO] Identity Toolkit API v1-rev20231027-2.0.0 v1-rev20231027-2.0.0 FAILURE [  5.907 s]
[INFO] Application Integration API v1alpha-rev20231112-2.0.0 v1alpha-rev20231112-2.0.0 FAILURE [  1.281 s]
[INFO] Data Lineage API v1-rev[20](https://github.com/googleapis/google-api-java-client-services/actions/runs/15397917088/job/43323308390?pr=27264#step:2:21)230901-2.0.0 v1-rev20230901-2.0.0 FAILURE [  1.337 s]