Switch PAT to GitHubApps

[chidozieononiwu]

This pull request updates the authentication method used for GitHub operations in pipeline scripts by replacing the azuresdk-github-pat variable with a new GH_TOKEN variable. It also adds a shared login step to standardize authentication. These changes help improve security and maintain consistency across the release notes and version updater pipelines.

Authentication updates:

• Replaced all instances of $(azuresdk-github-pat) with $(GH_TOKEN) for GitHub authentication in PowerShell scripts, arguments, and git remote URLs in both generate-releasenotes.yml and version-updater.yml. [1] [2] [3] [4]

Pipeline standardization:

• Added the /eng/common/pipelines/templates/steps/login-to-github.yml template to both generate-releasenotes.yml and version-updater.yml to centralize and standardize GitHub login steps. [1] [2]