Add changelog for recent yaml 1.2 change

Fixes: #6510

changelog/unreleased/config-yaml-1-2.md

@@ -0,0 +1,13 @@
+Change: YAML configuration files are restricted to yaml-1.2
+
+For parsing YAML based configuration files we utilize the gookit/config module.
+That module has dropped support for older variants of the YAML format. It now
+only supports the YAML 1.2 syntax.
+If you're using yaml configuration files, please make sure to update your files
+accordingly. The most significant change likely is that only the string `true`
+and `false` (including `TRUE`,`True`, `FALSE` and `False`) are now parsed as
+booleans. `Yes`, `On` and other values are not longer considered valid values
+for booleans.
+
+https://github.com/owncloud/ocis/issues/6510
+https://github.com/owncloud/ocis/pull/6493