docs(7-20-to-7-21): Add Feel-engine Upgrade Section

- Add documentation to the migration guide for the upgrade of feel-engine to 1.7.x version
- List identified breaking changes and full release notes change log

Related-to: camunda/camunda-bpm-platform#4100

content/update/minor/720-to-721/_index.md

@@ -23,6 +23,7 @@ This document guides you through the update from Camunda `7.20.x` to `7.21.0` an
 1. For developers: [Changed trigger order of built-in task listeners](#changed-trigger-order-of-built-in-task-listeners)
 1. For administrators and developers: [Cockpit's process definition like search changed to case-insensitive](#cockpit-s-process-definition-like-search-changed-to-case-insensitive)
 1. For developers: [External MDC properties are isolated from the engine's processing](#external-mdc-properties-are-isolated-from-engine-processing)
+1. For developers: [Feel-engine 1.7.x Upgrade](#feel-engine-1-7-upgrade)
 
 This guide covers mandatory migration steps and optional considerations for the initial configuration of new functionality included in Camunda 7.21.
 
@@ -98,4 +99,13 @@ This API is an internal API, which means it's **not** part of the public [REST A
 
 The previous behavior of _clearing the MDC tuples associated with Logging Context Parameters_ has been changed in this release. Starting from `7.21`, the new behavior preserves any logging context parameter entries found in the MDC to better isolate the engine processing from any external MDC user configuration.
 
-Use cases that require the legacy behaviour (clearing of the MDC logging context parameter tuples after the engine processing) can opt for clearing the MDC programmatically.
+Use cases that require the legacy behaviour (clearing of the MDC logging context parameter tuples after the engine processing) can opt for clearing the MDC programmatically.
+
+# Feel-engine 1.7 Upgrade
+
+The `7.21` release now supports `feel-engine 1.7.x`. This upgrade introduces breaking changes such as:
+
+- The invocation of a non-existing function returns null ([#692](https://github.com/camunda/feel-scala/issues/670))
+- Comparing different datatypes returns `false` instead of throwing exception ([#582](https://github.com/camunda/feel-scala/issues/582))
+
+For a detailed view of the change logs, checkout the `feel-engine 1.7.0` [Release notes](https://github.com/camunda/feel-scala/releases/tag/1.17.0)