Update functionbeat doc according to functionbeat deprecation planning

[jlind23]

Update functionbeat doc to clearly state that from now on this is deprecated.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @jlind23? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[pierrehilbert]

LGTM

[elasticmachine]

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[kilfoyle]

LGTM!

[kilfoyle]

@jlind23 what would you think of backporting this deprecation notice into 8.10? Then, I can update the docs build to freeze the Functionbeat docs at that level, and also move them down to the "Legacy Documentation" section of the docs home page.

[jlind23]

@kilfoyle sure! I'll backport it to 8.11 and 8.10 though