chore: define and parse default reviewers from team_members comment in CODEOWNERS

[shivanee-p]

Updates the PR auto-assignment workflow to identify default reviewers using a custom comment-based variable defined in the CODEOWNERS file, rather than querying members of the cloud-sdk-nodejs-team via the GitHub REST API.

• Custom team_members Variable in CODEOWNERS: Defined a group of default reviewers as a comment inside .github/CODEOWNERS
• Workflow Update: Updated the parsing script in .github/workflows/assign-reviewers.yml to fetch and parse CODEOWNERS. It scans comments for a line starting with team_members:, extracts the individual handles, and load-balances incoming PRs among them (excluding the PR author).
• Removed Team CC Fallback: Removed automatic routing to cloud-sdk-nodejs-team when load-balancing, ensuring that reviews are requested only from individual team members.

[gemini-code-assist]

Code Review

This pull request updates the .github/CODEOWNERS file to rename the default Node.js team to the 'cloud sdk nodejs' team and lists its team members in a comment. There are no review comments, so we have no feedback to provide.