feat(renovate): remove custom branchName for genai-toolbox updates

[SaxenaAnushka102]

Description

This PR updates the Renovate configuration to enable the auto-closing of older, superseded PRs for the genai-toolbox dependency.

Context

Previously, the packageRules for googleapis/genai-toolbox included a custom branchName (auto-update/toolbox-server-v{{newValue}}). While providing version-specific branches, this setup prevented Renovate's automated cleanup. Specifically, when a newer minor or patch version of genai-toolbox was released before an existing Renovate PR for an older version was merged, Renovate would not automatically close the superseded PR. This led to a build-up of obsolete Pull Requests.

This was caused by the direct usage of branchName in the configuration, which prevented Renovate from recognizing that the newer branch superseded the older one.

Changes

• This PR removes the custom "branchName" field from the packageRules block matching "googleapis/genai-toolbox".
• By removing the custom branch name, Renovate will now group these genai-toolbox minor and patch updates together with other non-major updates. Because "group:allNonMajor" is included in the extends, these updates will be consolidated into a single default branch, typically named renovate/all-minor-patch.
• Added configMigration: true to allow Renovate to suggest migrations for other deprecated fields in a separate PR.
• Although the branch names for genai-toolbox updates will now be more generic, the Pull Request titles, generated by commit messages using "commitMessageTopic": "MCP Toolbox server version in integration tests", will continue to clearly indicate that the PR is updating the MCP Toolbox server version.

This change will ensure a cleaner PR workflow for genai-toolbox updates going forward.