[Website]: Clarify failure store CCS support starts in Stack 9.4.0

[olgasyrova-dotcom]

Before you submit

• This issue is about a documentation page, flow, or piece of content.

Type of issue

Missing information

What documentation page or section is affected

https://www.elastic.co/docs/manage-data/data-store/data-streams/failure-store

What happened?

The "Cross Cluster Search compatibility" section should say that failure store CCS support is available starting in Stack 9.4.0.

I am adding a section-level applies_to tag because this version information applies only to this section, not the whole page:

#6381

Additional info

elastic/elasticsearch#139760

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