Remove the migrate tool (#42174)

This commit removes the deprecated migrate tool which was used to
migrate users from the file realm to native realm when the native realm
was first created.

Author: rjernst

docs/reference/commands/index.asciidoc

@@ -9,7 +9,6 @@ tasks from the command line:
 
 * <<certgen>>
 * <<certutil>>
-* <<migrate-tool>>
 * <<node-tool>>
 * <<saml-metadata>>
 * <<setup-passwords>>
@@ -21,7 +20,6 @@ tasks from the command line:
 
 include::certgen.asciidoc[]
 include::certutil.asciidoc[]
-include::migrate-tool.asciidoc[]
 include::node-tool.asciidoc[]
 include::saml-metadata.asciidoc[]
 include::setup-passwords.asciidoc[]

docs/reference/commands/migrate-tool.asciidoc

@@ -25,3 +25,11 @@ The `xpack.security.authz.store.roles.index.cache.max_size` and
 been removed. These settings have been redundant and deprecated
 since the 5.2 release of {es}.
 
+[float]
+[[migrate-tool-removed]]
+==== The `elasticsearch-migrate` tool has been removed
+
+The `elasticsearch-migrate` tool provided a way to convert file
+realm users and roles into the native realm. It has been deprecated
+since 7.2.0. Users and roles should now be created in the native
+realm directly.

docs/reference/migration/migrate_8_0/security.asciidoc

@@ -222,7 +222,6 @@ private static void verifyDefaultInstallation(Installation es, Distribution dist
             "elasticsearch-certgen",
             "elasticsearch-certutil",
             "elasticsearch-croneval",
-            "elasticsearch-migrate",
             "elasticsearch-saml-metadata",
             "elasticsearch-setup-passwords",
             "elasticsearch-sql-cli",

qa/vagrant/src/main/java/org/elasticsearch/packaging/util/Archives.java

@@ -244,7 +244,6 @@ private static void verifyDefaultInstallation(Installation es) {
             "elasticsearch-certgen",
             "elasticsearch-certutil",
             "elasticsearch-croneval",
-            "elasticsearch-migrate",
             "elasticsearch-saml-metadata",
             "elasticsearch-setup-passwords",
             "elasticsearch-sql-cli",