docs: update bonsai migration guide about YAML 1.2 (#5183)

In Bonsai (0.13.0) we started following the YAML 1.2 spec, and in Acorn
we are following the YAML 1.1 spec.
Since 0.13.6 we support YAML directives at the beginning of the file–
this allows users to choose the YAML spec version for backwards
compatibility.

This PR documents that in the Bonsai migration guide.

docs/guides/migrating-to-bonsai.md

@@ -41,6 +41,13 @@ Here is the list of breaking changes from Garden Acorn (0.12) to Bonsai (0.13).
   - `maven-container` (superseded by the `jib-container` plugin)
   - `npm-package` (the `exec` provider is a good replacement there)
   - `openfaas`
+- Bonsai follows the YAML 1.2 specification when reading Garden configuration files by default, and Acorn followed the 1.1 standard ([Summary of the differences](https://yaml.org/spec/1.2.2/ext/changes/#:~:text=The%20most%20significant%20difference%20between,counterparts%20are%20parsed%20as%20strings.)).
+  - To go back to using YAML 1.1 if you intend to make your config compatible with both Bonsai and Acorn, you can add a version directive at the top of the Garden configuration file:
+    ```YAML
+    %YAML 1.1
+    ---
+    ```
+  - Kubernetes still uses YAML 1.1 by default as of today ([Kubernetes issue](https://github.com/kubernetes/kubernetes/issues/34146)), and we continue to use YAML 1.1 when reading Kubernetes manifest files.
 
 ## Note before continuing