Add 2.1 upgrade notes also to devel branch

docs/simplesamlphp-upgrade-notes-2.1.md

+# Upgrade notes for SimpleSAMLphp 2.1
+
+SimpleSAMLphp 2.1 is a minor new release which bumps some
+of the minimum requirements to be able to run SimpleSAMLphp.
+The following changes are relevant for installers and/or developers.
+
+## Software requirements
+
+- The minimum PHP version required is now PHP 8.0.
+- Dropped support for Symfony 5.4.
+
+## Two builds
+
+As of SimpleSAMLphp 2.1 two builds are created for every release.
+The 'slim' build is a lightweight build without any modules other than the core modules installed (alike the 2.0 build).
+A new 'full' build is added that will come with the most used modules pre-installed.

docs/simplesamlphp-upgrade-notes.md

@@ -3,6 +3,7 @@
 See the following pages for important information for users upgrading
 from older versions of SimpleSAMLphp:
 
+* [Upgrade notes for version 2.1](simplesamlphp-upgrade-notes-2.1)
 * [Upgrade notes for version 2.0](simplesamlphp-upgrade-notes-2.0)
 * [Upgrade notes for version 1.19](simplesamlphp-upgrade-notes-1.19)
 * [Upgrade notes for version 1.18](simplesamlphp-upgrade-notes-1.18)