[Docs] Reset release files for 9.0 (#199519)

This PR resets the release notes, upgrade notes, and what's new for 9.0.
It also cleans up a few references/files that were focusing on migration
to 8.0

Some more PRs will happen to prepare the rest of the docs for v9

Closes: elastic/platform-docs-team#564

docs/developer/architecture/development-visualize-index.asciidoc

@@ -19,7 +19,7 @@ We would recommend waiting until later in `7.x` to upgrade your plugins if possi
 If you would like to keep up with progress on the visualizations plugin in the meantime,
 here are a few resources:
 
-* The <<breaking-changes-8.0.0,breaking changes>> documentation, where we try to capture any changes to the APIs as they occur across minors.
+* The <<breaking-changes-9.0.0,breaking changes>> documentation, where we try to capture any changes to the APIs as they occur across minors.
 * link:https://github.com/elastic/kibana/issues/44121[Meta issue] which is tracking the move of the plugin to the new {kib} platform
 * Our link:https://www.elastic.co/blog/join-our-elastic-stack-workspace-on-slack[Elastic Stack workspace on Slack].
 * The {kib-repo}blob/{branch}/src/plugins/visualizations[source code], which will continue to be

docs/migration.asciidoc

@@ -6,9 +6,9 @@
 This section discusses the changes that you need to be aware of when migrating
 your application from one version of Kibana to another.
 
-* <<breaking-changes-8.0>>
+* <<breaking-changes-9.0>>
 
 See also <<whats-new>> and <<release-notes>>. 
 --
 
-include::migration/migrate_8_0.asciidoc[]
+include::migration/migrate_9_0.asciidoc[]

docs/migration/migrate_9_0.asciidoc

@@ -0,0 +1,18 @@
+[[breaking-changes-9.0]]
+== Breaking changes in 9.0
+++++
+<titleabbrev>9.0</titleabbrev>
+++++
+
+This section discusses the changes that you need to be aware of when migrating
+your application to Kibana 9.0.
+
+See also <<whats-new>> and <<release-notes>>.
+
+* <<breaking_90_setting_changes>>
+
+
+[float]
+[[breaking_90_setting_changes]]
+=== Settings changes
+

docs/redirects.asciidoc

@@ -418,10 +418,6 @@ This page has been deleted. Refer to <<machine-learning-api-sync>>.
 
 This page has been deleted. Refer to <<alerting-getting-started>>.
 
-[role="exclude",id="enhancements-and-bug-fixes-v8.10.0"]
-== Enhancements and bug fixes for 8.10.0
-
-This content has moved. Refer to <<enhancements-and-bug-fixes-v8.10.0-revised>> for 8.10.0.
 
 [role="exclude",id="gen-ai-action-type"]
 == Generative AI connector and action

docs/setup/upgrade/upgrade-standard.asciidoc

@@ -30,7 +30,7 @@ Different versions of {kib} running against the same {es} index, such as during
 +
 
 Make sure you remove or update any configurations
-that are indicated in the <<breaking-changes-8.0.0,breaking changes>> documentation
+that are indicated in the <<breaking-changes-9.0.0,breaking changes>> documentation
 otherwise {kib} will fail to start.
 --
 . Upgrade any plugins by removing the existing plugin and reinstalling the
@@ -51,7 +51,7 @@ and becomes a new instance in the monitoring data.
 --
 . Copy the files from the `config` directory from your old installation to your
   new installation. Make sure you remove or update any configurations that are
-  indicated in the <<breaking-changes-8.0.0,breaking changes>> documentation
+  indicated in the <<breaking-changes-9.0.0,breaking changes>> documentation
   otherwise {kib} will fail to start.
 . Copy the files from the `data` directory from your old installation to your
   new installation.