chore(linter): Migrate Redocly plugin to v2 and extract as separate package

[maxedenharter0507]

• Migrates the Redocly linting plugin from @redocly/openapi-core v1 to v2: plugin now exports a default PluginCreator function, config uses async createConfig(), and all deep imports replaced with the public API entry point
• Extracts the plugin into a new, separately publishable npm package @otto-de/api-guidelines-redocly2-ruleset using npm workspaces, while keeping both packages in this repo
• Both packages are always released at the same version via the shared release.yml workflow using a single git tag
• Adds build:all, tsc:all, and test:all root scripts to operate across all workspaces and the root in one command; CI updated accordingly

[plemech23]

LGTM