Remove "warning" about "fulltextsearch"

[Firesphere]

The "silverstripe-labs" fulltextsearch module is incredibly outdated, and relies on a very outdated not-vendorized library. The security and stability of this module is questionable at best, and should not be suggested

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[GuySartorelli]

That module had its supported status removed for CMS 5 so makes sense to remove this reference to it. Thanks for submitting.

Might make sense to add a new comment here advising developers to look for third-party modules providing integrations with search services but that can always be done in a follow-up PR.

[GuySartorelli]

Note the commit message was missing the DOC prefix but I added that on merge. Just an FYI for next time.