Update upgrade log (#207)

Co-authored-by: Marko Ivančić <[email protected]>

Author: cicnavi

UPGRADE.md

@@ -1,3 +1,7 @@
+# Version 3 to 4
+- PHP version requirement was bumped to v8.0 to enable updating important dependant packages like 'league/oauth2-server'
+  which has already moved to PHPv8 between their minor releases.
+
 # Version 2 to 3
  - Module code was refactored to make it compatible with SimpleSAMLphp v2
  - Default key name was changed from oidc_module.pem to oidc_module.key. If you don't set custom