DOC-3374: Remove Step-by-Step Migration Guides (4→7, 5→7, 6→7). (#3991)

Author: kemister85

modules/ROOT/nav.adoc

@@ -178,10 +178,6 @@
 **** xref:migration-from-5x-to-8x.adoc[Migrating from 5 to 8]
 **** xref:migration-from-6x-to-8x.adoc[Migrating from 6 to 8]
 **** xref:migration-from-7x.adoc[Migrating from 7 to 8]
-*** Step-by-Step Migration Guides
-**** xref:migration-from-4x.adoc[Migrating from 4 to 7]
-**** xref:migration-from-5x.adoc[Migrating from 5 to 7]
-**** xref:migration-from-6x.adoc[Migrating from 6 to 7]
 *** xref:migration-from-froala.adoc[Migrating from Froala]
 ** xref:generate-rsa-key-pairs.adoc[Generate public key pairs]
 ** xref:understanding-editor-loads.adoc[Understanding editor loads]

modules/ROOT/pages/migration-from-4x-to-8x.adoc

@@ -769,7 +769,6 @@ To make your upgrade smooth, check the following version-specific migration guid
 
 * link:https://www.tiny.cloud/docs/tinymce/5/migration-from-4x/[Migrating from {productname} 4 to {productname} 5]
 * link:https://www.tiny.cloud/docs/tinymce/6/migration-from-5x/[Migrating from {productname} 5 to {productname} 6]
-* xref:migration-from-6x.adoc[Migrating from {productname} 6 to {productname} 7]
 * xref:migration-from-7x.adoc[Migrating from {productname} 7 to {productname} 8]
 
 These include deeper configuration notes, plugin replacements, and examples.

modules/ROOT/pages/migration-from-4x.adoc

@@ -665,7 +665,6 @@ To make your upgrade smooth, check the following version-specific migration guid
 
 * link:https://www.tiny.cloud/docs/tinymce/5/migration-from-4x/[Migrating from {productname} 4 to {productname} 5]
 * link:https://www.tiny.cloud/docs/tinymce/6/migration-from-5x/[Migrating from {productname} 5 to {productname} 6]
-* xref:migration-from-6x.adoc[Migrating from {productname} 6 to {productname} 7]
 * xref:migration-from-7x.adoc[Migrating from {productname} 7 to {productname} 8]
 
 These include deeper configuration notes, plugin replacements, and examples.