Move instructor embedders integration to staging

[julian-risch]

We should move the instructor embedders integration to staging for the following reasons

• long list of direct dependencies, which are extremely difficult to pin in a meaningful way. InstructorEmbedding PyPI package does not properly declare its own mandatory dependencies.
• InstructorEmbedding project is in an almost abandoned state: it seems that the current maintainer of https://github.com/xlang-ai/instructor-embedding can commit on main but cannot release new versions of the package on PyPI
• low usage of this integration (<100 downloads per month)

https://docs.haystack.deepset.ai/docs/breaking-change-policy#discontinuing-an-integration

Related #1796