[8.1] [DOCS] Removes legacy plugins (elastic#149545) (elastic#149761)

# Backport

This will backport the following commits from `main` to `8.1`:
- [[DOCS] Removes legacy plugins
(elastic#149545)](elastic#149545)

docs/developer/architecture/kibana-platform-plugin-api.asciidoc

@@ -176,7 +176,6 @@ of Kibana.
 |store session data in the LocalStorage when the user navigates away from {kib}, etc.
 |===
 
-There is no equivalent behavior to `start` or `stop` in legacy plugins.
 Conversely, there is no equivalent to `uiExports` in Kibana Platform plugins.
 As a general rule of thumb, features that were registered via `uiExports` are
 now registered during the `setup` phase. Most of everything else should move
@@ -339,9 +338,3 @@ interface where the property name is the plugin's ID.
 the second argument to the plugin.
 <3> Notice that the type for the setup and start lifecycles are different. Plugin lifecycle
 functions can only access the APIs that are exposed _during_ that lifecycle.
-
-=== Migrating legacy plugins
-
-In Kibana 7.10, support for legacy plugins was removed. See
-<<migrating-legacy-plugins>> for detailed information on how to convert existing
-legacy plugins to this new API.

docs/developer/getting-started/development-plugin-resources.asciidoc

@@ -48,13 +48,6 @@ TypeScript code is automatically converted into JavaScript during development,
 but not in the distributable version of {kib}. If you use the
 {kib-repo}blob/{branch}/packages/kbn-plugin-helpers[@kbn/plugin-helpers] to build your plugin, then your `.ts` and `.tsx` files will be permanently transpiled before your plugin is archived. If you have your own build process, make sure to run the TypeScript compiler on your source files and ship the compilation output so that your plugin will work with the distributable version of {kib}.
 
-[discrete]
-=== {kib} platform migration guide
-
-<<migrating-legacy-plugins, This guide>>
-provides an action plan for moving a legacy plugin to the new platform.
-<<migrating-legacy-plugins-examples, Provides>> migration examples for the legacy core services.
-
 [discrete]
 === Externally developed plugins
 

docs/developer/plugin/index.asciidoc

@@ -10,15 +10,11 @@ Most developers who contribute code directly to the {kib} repo are writing code
 start. However, there are a few differences when developing plugins outside the {kib} repo. These differences are covered here.
 
 * <<plugin-tooling>>
-* <<migrating-legacy-plugins>>
-* <<migrating-legacy-plugins-examples>>
 * <<external-plugin-functional-tests>>
 * <<external-plugin-localization>>
 * <<testing-kibana-plugin>>
 
 include::plugin-tooling.asciidoc[leveloffset=+1]
-include::migrating-legacy-plugins.asciidoc[leveloffset=+1]
-include::migrating-legacy-plugins-examples.asciidoc[leveloffset=+1]
 include::external-plugin-functional-tests.asciidoc[leveloffset=+1]
 include::external-plugin-localization.asciidoc[leveloffset=+1]
-include::testing-kibana-plugin.asciidoc[leveloffset=+1]
+include::testing-kibana-plugin.asciidoc[leveloffset=+1]