Docs: Specify an ID in migration docs (#41501)

We link to these migraiton docs but we don't specify the id. This
isn't great practice in general and is preventing us from migrating to
Asciidoctor because it generates ids in a slightly different way.

Author: nik9000

docs/reference/migration/migrate_7_0/cluster.asciidoc

@@ -10,6 +10,7 @@
 // end::notable-breaking-changes[]
 
 [float]
+[[_literal_literal_is_no_longer_allowed_in_cluster_name]]
 ==== `:` is no longer allowed in cluster name
 
 Due to cross-cluster search using `:` to separate a cluster and index name,

docs/reference/migration/migrate_7_0/indices.asciidoc

@@ -14,6 +14,7 @@ Starting with 7.0.0, the default is now one shard per index.
 //end::notable-breaking-changes[]
 
 [float]
+[[_literal_literal_is_no_longer_allowed_in_index_name]]
 ==== `:` is no longer allowed in index name
 
 Due to cross-cluster search using `:` to separate a cluster and index name,