Update "Upgrading From" section to additionally use "2.x"

See gh-43123

Co-authored-by: Moritz Halbritter <moritz.halbritter@broadcom.com>

spring-boot-project/spring-boot-docs/src/docs/asciidoc/documentation/upgrading.adoc

@@ -5,7 +5,8 @@ You should always ensure that you are running a {github-wiki}/Supported-Versions
 
 Depending on the version that you are upgrading to, you can find some additional tips here:
 
-* *From 1.x:* <<upgrading#upgrading.from-1x, Upgrading from 1.x>>
+* *From 1.x:* <<upgrading#upgrading.from-1x, Upgrading from 1.x to 2.x>>
+* *From 2.x:* <<upgrading#upgrading.from-2x, Upgrading from 2.x to 3.x>>
 * *To a new feature release:* <<upgrading#upgrading.to-feature, Upgrading to New Feature Release>>
 * *Spring Boot CLI:* <<upgrading#upgrading.cli, Upgrading the Spring Boot CLI>>
 

spring-boot-project/spring-boot-docs/src/docs/asciidoc/upgrading.adoc

@@ -10,6 +10,8 @@ If you are more than one release behind, please make sure that you also review t
 
 include::upgrading/from-1x.adoc[]
 
+include::upgrading/from-2x.adoc[]
+
 include::upgrading/to-feature.adoc[]
 
 include::upgrading/cli.adoc[]

spring-boot-project/spring-boot-docs/src/docs/asciidoc/upgrading/from-1x.adoc

@@ -1,5 +1,5 @@
 [[upgrading.from-1x]]
 == Upgrading From 1.x
 
-If you are upgrading from the `1.x` release of Spring Boot, check the {github-wiki}/Spring-Boot-2.0-Migration-Guide["`migration guide`" on the project wiki] that provides detailed upgrade instructions.
+If you are upgrading from the `1.x` release of Spring Boot, check the {github-wiki}/Spring-Boot-2.0-Migration-Guide["`migration guide`" on the project wiki] that provides detailed upgrade instructions to upgrade to Spring Boot 2.x.
 Check also the {github-wiki}["`release notes`"] for a list of "`new and noteworthy`" features for each release.

spring-boot-project/spring-boot-docs/src/docs/asciidoc/upgrading/from-2x.adoc

@@ -0,0 +1,5 @@
+[[upgrading.from-2x]]
+== Upgrading From 2.x
+
+If you are upgrading from the `2.x` release of Spring Boot, check the {github-wiki}/Spring-Boot-3.0-Migration-Guide["`migration guide`" on the project wiki] that provides detailed upgrade instructions.
+Check also the {github-wiki}["`release notes`"] for a list of "`new and noteworthy`" features for each release.