9.0: update docs, versions and changelogs (#15481)

* [Release] update version 9.1.0

* [Release] update changelogs for 9.0 release

---------

Co-authored-by: obltmachine <[email protected]>
Co-authored-by: Victor Martinez <[email protected]>
Co-authored-by: Carson Ip <[email protected]>

CHANGELOG.asciidoc

@@ -1,4 +1,5 @@
 // tag::list[]
+* <<apm-release-notes-9.0>>
 * <<apm-release-notes-8.18>>
 * <<apm-release-notes-8.17>>
 * <<apm-release-notes-8.16>>
@@ -21,6 +22,8 @@
 // end::list[]
 
 // tag::includes[]
+include::./changelogs/head.asciidoc[]
+include::./changelogs/9.0.asciidoc[]
 include::./changelogs/8.18.asciidoc[]
 include::./changelogs/8.17.asciidoc[]
 include::./changelogs/8.16.asciidoc[]

changelogs/9.0.asciidoc

@@ -0,0 +1,26 @@
+[[apm-release-notes-9.0]]
+== APM version 9.0
+* <<apm-release-notes-9.0.0>>
+
+[float]
+[[apm-release-notes-9.0.0]]
+=== APM version 9.0.0
+
+https://github.com/elastic/apm-server/compare/v\...v9.0.0[View commits]
+
+[float]
+==== Bug fixes
+- Fix overflow in validation of `apm-server.agent.config.cache.expiration` on 32-bit architectures {pull}15216[15216]
+
+[float]
+==== Breaking Changes
+
+[float]
+==== Deprecations
+
+[float]
+==== Intake API Changes
+
+[float]
+==== Added
+- Tail-based sampling: Storage layer is rewritten to use Pebble database instead of BadgerDB. The new implementation offers a substantial throughput increase while consuming significantly less memory. Disk usage is lower and more stable. See PR for benchmark details. {pull}15235[15235]

changelogs/head.asciidoc

@@ -1,7 +1,7 @@
 [[release-notes-head]]
 == APM version HEAD
 
-https://github.com/elastic/apm-server/compare/8.18\...8.x[View commits]
+https://github.com/elastic/apm-server/compare/9.0\...main[View commits]
 
 [float]
 ==== Breaking Changes

cmd/intake-receiver/version.go

@@ -18,4 +18,4 @@
 package main
 
 // version matches the APM Server's version
-const version = "9.0.0"
+const version = "9.1.0"

internal/version/version.go

@@ -18,4 +18,4 @@
 package version
 
 // Version holds the APM Server version.
-const Version = "9.0.0"
+const Version = "9.1.0"