Fix Publish to Java Dev Feed

[alzimmermsft]

Description

Fixes a bug that broke Publish to Java Dev Feed.

This issue arose due to a loose usage of /eng/versioning/external_dependencies.txt to resolve the version of maven-gpg-plugin. This change hardcodes the version of maven-gpg-plugin in the Publish-MavenPackages.ps1 script as it is an extremely unique usage of a dependency and should be treated differently than our other external dependencies. Doing this prevents conflicts with cleanup scripts that won't know about this usage and prevents cases where this could have been broken by accidental version changes that wouldn't be noticed in the PR as this logic is only invoked during release.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[samvaity]

We will just have to keep updating it as it won't be read from the dependencies file anymore.

[alzimmermsft]

/check-enforcer override