Add to the 7.17 to 9.x upgrade doc info about Security solution

[eedugon]

I didn't realize during the creation of the 7.x to 9.x upgrade doc that we had these 2 documents on the Security Solution.

https://www.elastic.co/guide/en/security/8.19/upgrade-intro.html
https://www.elastic.co/guide/en/security/8.19/upgrade-7.17-8x.html

They offer interesting and relevant details for Elastic Security / SIEM / Endpoint Security related use cases.

We could also check if there are other similar docs for other Solutions (Observability mainly, as Search was probably covered with the comments about Enterprise Search and the migration doc).

cc: @shainaraskas / @ppf2

[ppf2]

Yeah, the big one for Search is the removal of legacy Enterprise Search products and features that depend on Enterprise Search from 9.0+. But I think there are well covered in docs as well as the upgrade assistant.

[eedugon]

@ppf2 , thanks! that doc is already linked in the guide we have recently created

Anyway whatever else you think we may have missed let us know!