Add upgrade guide for 4.21

Author: davsclaus

docs/user-manual/modules/ROOT/pages/camel-4x-upgrade-guide-4_21.adoc

@@ -0,0 +1,15 @@
+= Apache Camel 4.x Upgrade Guide
+
+This document is for helping you upgrade your Apache Camel application
+from Camel 4.x to 4.y. For example, if you are upgrading Camel 4.0 to 4.2, then you should follow the guides
+from both 4.0 to 4.1 and 4.1 to 4.2.
+
+[NOTE]
+====
+https://github.com/apache/camel-upgrade-recipes/[The Camel Upgrade Recipes project] provides automated assistance for some common migration tasks.
+Note that manual migration is still required.
+See the xref:camel-upgrade-recipes-tool.adoc[documentation] page for details.
+====
+
+== Upgrading Camel 4.20 to 4.21
+

docs/user-manual/modules/ROOT/pages/camel-4x-upgrade-guide.adoc

@@ -12,23 +12,24 @@ You can find the upgrade guide for each release in the following pages:
 - xref:camel-4x-upgrade-guide-4_1.adoc[Upgrade guide 4.0 -> 4.1]
 - xref:camel-4x-upgrade-guide-4_2.adoc[Upgrade guide 4.1 -> 4.2]
 - xref:camel-4x-upgrade-guide-4_3.adoc[Upgrade guide 4.2 -> 4.3]
-- xref:camel-4x-upgrade-guide-4_4.adoc[Upgrade guide 4.3 -> 4.4]
+- xref:camel-4x-upgrade-guide-4_4.adoc[Upgrade guide 4.3 -> 4.4 LTS]
 - xref:camel-4x-upgrade-guide-4_5.adoc[Upgrade guide 4.4 -> 4.5]
 - xref:camel-4x-upgrade-guide-4_6.adoc[Upgrade guide 4.5 -> 4.6]
 - xref:camel-4x-upgrade-guide-4_7.adoc[Upgrade guide 4.6 -> 4.7]
-- xref:camel-4x-upgrade-guide-4_8.adoc[Upgrade guide 4.7 -> 4.8]
+- xref:camel-4x-upgrade-guide-4_8.adoc[Upgrade guide 4.7 -> 4.8 LTS]
 - xref:camel-4x-upgrade-guide-4_9.adoc[Upgrade guide 4.8 -> 4.9]
-- xref:camel-4x-upgrade-guide-4_10.adoc[Upgrade guide 4.9 -> 4.10]
+- xref:camel-4x-upgrade-guide-4_10.adoc[Upgrade guide 4.9 -> 4.10 LTS]
 - xref:camel-4x-upgrade-guide-4_11.adoc[Upgrade guide 4.10 -> 4.11]
 - xref:camel-4x-upgrade-guide-4_12.adoc[Upgrade guide 4.11 -> 4.12]
 - xref:camel-4x-upgrade-guide-4_13.adoc[Upgrade guide 4.12 -> 4.13]
-- xref:camel-4x-upgrade-guide-4_14.adoc[Upgrade guide 4.13 -> 4.14]
+- xref:camel-4x-upgrade-guide-4_14.adoc[Upgrade guide 4.13 -> 4.14 LTS]
 - xref:camel-4x-upgrade-guide-4_15.adoc[Upgrade guide 4.14 -> 4.15]
 - xref:camel-4x-upgrade-guide-4_16.adoc[Upgrade guide 4.15 -> 4.16]
 - xref:camel-4x-upgrade-guide-4_17.adoc[Upgrade guide 4.16 -> 4.17]
-- xref:camel-4x-upgrade-guide-4_18.adoc[Upgrade guide 4.17 -> 4.18]
+- xref:camel-4x-upgrade-guide-4_18.adoc[Upgrade guide 4.17 -> 4.18 LTS]
 - xref:camel-4x-upgrade-guide-4_19.adoc[Upgrade guide 4.18 -> 4.19]
 - xref:camel-4x-upgrade-guide-4_20.adoc[Upgrade guide 4.19 -> 4.20]
+- xref:camel-4x-upgrade-guide-4_21.adoc[Upgrade guide 4.20 -> 4.21]
 
 [NOTE]
 ====