Add Fleet Server 8.19.16 release notes (#7091)

* add the 8.19.16 Fleet Server release notes

* Apply suggestions from code review

Co-authored-by: Visha Angelova <91186315+vishaangelova@users.noreply.github.com>

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Co-authored-by: ebeahan <7226265+ebeahan@users.noreply.github.com>
Co-authored-by: Eric Beahan <eric.beahan@elastic.co>
Co-authored-by: Visha Angelova <91186315+vishaangelova@users.noreply.github.com>

Author: 3 people

changelog/8.19.16.asciidoc

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+// begin 8.19.16 relnotes
+
+[[release-notes-8.19.16]]
+==  8.19.16
+
+Review important information about the  8.19.16 release.
+
+[discrete]
+[[security-updates-8.19.16]]
+=== Security updates
+
+
+
+
+* Fix missing `authAgent` in `UploadChunk`. https://github.com/elastic/fleet-server/pull/7020[#7020] 
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+// end 8.19.16 relnotes

changelog/8.19.16.yaml

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+version: 8.19.16
+entries:
+    - kind: security
+      summary: fix missing authagent in uploadchunk
+      description: |
+        Validate that the uploading agent of the chunk has the permissions to write that chunk.
+      component: fleet-server
+      pr:
+        - https://github.com/elastic/fleet-server/pull/7020
+      issue: []
+      impact: ""
+      action: ""
+      timestamp: 1778534244
+      file:
+        name: 1778534244-fix-missing-authagent-in-uploadchunk.yaml
+        checksum: 8d4dcc68c12c238c6424e294daf18359f2e154c4