chore(deps): update rust docker tag to v1.91

[elastic-renovate-prod]

This PR contains the following updates:

Package	Type	Update	Change
rust	stage	minor	1.89 -> 1.91

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This PR has been generated by Renovate Bot.

[JoshMock]

⚠️ Deprecation warning

This MCP server is deprecated and will only receive critical security updates going forward. It has been superseded by Elastic Agent Builder's MCP endpoint, which is available in Elastic 9.2.0+ and Elasticsearch Serverless projects.