Enterprsie search, elasticsearch and connectors release notes for 8.18.6 and 8.19.3

[charlotte-hoblik]

Create release notes for:

• 8.18.6
  • Search team issue: https://github.com/elastic/search-team/issues/10756
  • Docs team issue: https://github.com/elastic/dev/issues/3271
• 8.19.3
  • Search team issue: https://github.com/elastic/search-team/issues/10795
  • Docs team issue: https://github.com/elastic/dev/issues/3272

Deadline: 2025.08.27

Products:

• Enterprise search:

  • 8.18.6: https://github.com/elastic/enterprise-search-pubs/pull/4819
  • 8.19.3: https://github.com/elastic/enterprise-search-pubs/pull/4820

• Elasticsearch:

  • 8.18.6: Add release notes for v8.18.6 release elasticsearch#133551
  • 8.19.3: Add release notes for v8.19.3 release elasticsearch#133548

• Connectors:

  • 8.18.6: [DOCS]: Release notes search connectors 8.18.6 elasticsearch#133635
  • 8.19.3: [DOCS]: Release notes for search connectors 8.19.3 elasticsearch#133637

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• create backports