Prep docs for v9 (elastic#115375)

This commit prepares the documentation for version 9. 

Some of the automation generates docs that are not correct for version 9. 
The content has been commented out with a reference to an internal issue 
for us to address before this documentation is used.

Author: jakelandis

docs/reference/migration/index.asciidoc

@@ -1,40 +1,6 @@
 include::migration_intro.asciidoc[]
 
-* <<migrating-8.17,Migrating to 8.17>>
-* <<migrating-8.16,Migrating to 8.16>>
-* <<migrating-8.15,Migrating to 8.15>>
-* <<migrating-8.14,Migrating to 8.14>>
-* <<migrating-8.13,Migrating to 8.13>>
-* <<migrating-8.12,Migrating to 8.12>>
-* <<migrating-8.11,Migrating to 8.11>>
-* <<migrating-8.10,Migrating to 8.10>>
-* <<migrating-8.9,Migrating to 8.9>>
-* <<migrating-8.8,Migrating to 8.8>>
-* <<migrating-8.7,Migrating to 8.7>>
-* <<migrating-8.6,Migrating to 8.6>>
-* <<migrating-8.5,Migrating to 8.5>>
-* <<migrating-8.4,Migrating to 8.4>>
-* <<migrating-8.3,Migrating to 8.3>>
-* <<migrating-8.2,Migrating to 8.2>>
-* <<migrating-8.1,Migrating to 8.1>>
-* <<migrating-8.0,Migrating to 8.0>>
+* <<migrating-9.0,Migrating to 9.0>>
 
-include::migrate_8_17.asciidoc[]
-include::migrate_8_16.asciidoc[]
-include::migrate_8_15.asciidoc[]
-include::migrate_8_14.asciidoc[]
-include::migrate_8_13.asciidoc[]
-include::migrate_8_12.asciidoc[]
-include::migrate_8_11.asciidoc[]
-include::migrate_8_10.asciidoc[]
-include::migrate_8_9.asciidoc[]
-include::migrate_8_8.asciidoc[]
-include::migrate_8_7.asciidoc[]
-include::migrate_8_6.asciidoc[]
-include::migrate_8_5.asciidoc[]
-include::migrate_8_4.asciidoc[]
-include::migrate_8_3.asciidoc[]
-include::migrate_8_2.asciidoc[]
-include::migrate_8_1.asciidoc[]
-include::migrate_8_0.asciidoc[]
+include::migrate_9_0.asciidoc[]