Update Gradle to 7.4 and pubsub client (and dependencies) to 1.120.13

[andsel]

Release notes

Updates PubSub dependencies forcing this plugin to be compatible only with Logstash >= 8.2.0

What does this PR do?

Update Google PubSub client (and transitive dependencies) to 1.115.1 from 1.37.1

Why is it important/What is the impact to the user?

• Keeps libraries updated
• Removes 7.x branch testing from Travis matrix test

Checklist

• [x] My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
• [ ] I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

How to test this PR locally

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Closes https://github.com/elastic/logstash/issues/14474