Migrate from opentelemetrybot to otelbot

[Copilot]

This PR migrates GitHub workflows from using the opentelemetrybot user account to the otelbot GitHub app, as requested in the migration initiative.

Key Changes

Token Migration

• Replaced ${{ secrets.OPENTELEMETRYBOT_GITHUB_TOKEN }} with ${{ steps.otelbot-token.outputs.token }}
• Added otelbot token generation step using actions/create-github-app-token@df432ceedc7162793a195dd1713ff69aefc7379e before first usage in both workflows

Workflow Architecture Changes

The create-or-update-release-pr.yml workflow required significant restructuring because GitHub apps don't have forks:

Before: Used opentelemetrybot's fork of the repository

• Forked the repo using gh repo fork
• Checked out opentelemetrybot/opentelemetry-js
• Synced with upstream and force-pushed to fork
• Created PRs from the fork

After: Works directly on the main repository

• Removed fork step entirely
• Pushes directly to otelbot/release/next-version branch on main repo
• Uses secrets.GITHUB_TOKEN for branch operations (otelbot app lacks push permissions)
• Uses otelbot token for PR creation (to ensure workflows trigger properly)

Configuration Updates

• Updated git config from opentelemetrybot/107717825+opentelemetrybot@users.noreply.github.com to otelbot/197425009+otelbot@users.noreply.github.com
• Changed RELEASE_PR_OWNER from opentelemetrybot to otelbot
• Updated permissions to include contents: write for branch operations
• Updated documentation reference from @opentelemetrybot to @otelbot[bot] (GitHub apps have [bot] suffix)

Security Considerations

The new approach properly separates concerns:

• secrets.GITHUB_TOKEN handles branch push operations where otelbot lacks permissions
• otelbot token handles PR operations where workflow triggering is required

Files Modified

• .github/workflows/survey-on-merged-pr.yml - Token migration only
• .github/workflows/create-or-update-release-pr.yml - Complete architecture change
• doc/contributing/releasing.md - Documentation update

Fixes #54.

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