Docs: Pin two IDs in the rest client (#40785)

We generate two pages with "funny" names:
* _changing_the_client_8217_s_initialization_code.html
* _changing_the_application_8217_s_code.html

The leading `_` comes from us not specifying the name of the page. The
`8217` comes about because of the single quote character. This is a
funny name, but it is the name that we have so we shouldn't change it
without putting in a redirect.

We're looking at switching these docs from being built with the
no-longer-maintained AsciiDoc project to being built with the
actively-maintained Asciidoctor project. Asciidoctor Doesn't include the
`8217`s in the generated ids. That is *better*, but we don't really want
to change the pages. Ultimately we'd prefer none of our pages start with
`_`, but that is a problem for a different time.

Anyway, this pins the ids to their "funny" id so it won't change when we
switch to Asciidoctor. We'll remove it later, when we have more fine
control of our redirects.

Author: nik9000

docs/java-rest/high-level/migration.asciidoc

@@ -45,6 +45,8 @@ The <<java-rest-high-getting-started,Getting Started>> page shows
  <<java-rest-high-getting-started-dependencies, dependencies>> brought by the
  high-level client.
 
+// This ID is bad but it is the one we've had forever.
+[[_changing_the_client_8217_s_initialization_code]]
 === Changing the client's initialization code
 
 The `TransportClient` is typically initialized as follows:
@@ -94,6 +96,8 @@ must be replaced with:
 include-tagged::{doc-tests}/MainDocumentationIT.java[rest-high-level-client-close]
 --------------------------------------------------
 
+// This ID is bad but it is the one we've had forever.
+[[_changing_the_application_8217_s_code]]
 === Changing the application's code
 
 The `RestHighLevelClient` supports the same request and response objects