Stops getting new vscode GitHub sessions, creates new sessions via GKDev

[sergeibbb]

Description

Implements #4881

Stops requesting for GitHub session retrieval via VSCode built-in sessions. Instead, if there is no an existing available for GitLens session in VSCode, then it always goes to the GKDev flow.

Checklist

• I have followed the guidelines in the Contributing document
• My changes follow the coding style of this project
• My changes build without any errors or warnings
• My changes have been formatted and linted
• My changes include any required corresponding changes to the documentation (including CHANGELOG.md and README.md)
• My changes have been rebased and squashed to the minimal number (typically 1) of relevant commits
• My changes have a descriptive commit message with a short title, including a Fixes $XXX - or Closes #XXX - prefix to auto-close the issue that your PR addresses

[augmentcode]

🤖 Augment PR Summary

Summary: Adjusts GitHub authentication so GitLens no longer requests new VS Code built-in GitHub sessions, falling back to the GKDev/cloud flow when no existing session is available.

Changes:

• Removes the built-in forceNewSession request path for GitHub sessions and only attempts a silent built-in lookup
• Updates the changelog to document the shift from VS Code session creation to the GKDev flow

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[augmentcode]

Review completed. 2 suggestions posted.

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