[DOCS] Adding a known issue for 8.13 and 8.12 (#180971)

## Summary

This PR adds a known issue to the 8.13 and 8.12 release notes regarding
the `allow_auto_create` field incorrectly set to `false` in the Index
Templates form.

<img width="769" alt="Screenshot 2024-04-17 at 08 14 02"
src="4bba4879-3522-4fb4-81de-8da34b8794fd">


Original issue: https://github.com/elastic/kibana/issues/177938
PR with a fix: https://github.com/elastic/kibana/pull/178321 (The PR was
backported to 8.13 but unfortunately the build failed on the backport PR
and we missed that. So the fix should come out in the next scheduled
release, which is 8.14).


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### Risk Matrix

Delete this section if it is not applicable to this PR.

Before closing this PR, invite QA, stakeholders, and other developers to
identify risks that should be tested prior to the change/feature
release.

When forming the risk matrix, consider some of the following examples
and how they may potentially impact the change:

| Risk | Probability | Severity | Mitigation/Notes |

|---------------------------|-------------|----------|-------------------------|
| Multiple Spaces&mdash;unexpected behavior in non-default Kibana Space.
| Low | High | Integration tests will verify that all features are still
supported in non-default Kibana Space and when user switches between
spaces. |
| Multiple nodes&mdash;Elasticsearch polling might have race conditions
when multiple Kibana nodes are polling for the same tasks. | High | Low
| Tasks are idempotent, so executing them multiple times will not result
in logical error, but will degrade performance. To test for this case we
add plenty of unit tests around this logic and document manual testing
procedure. |
| Code should gracefully handle cases when feature X or plugin Y are
disabled. | Medium | High | Unit tests will verify that any feature flag
or plugin combination still results in our service operational. |
| [See more potential risk
examples](https://github.com/elastic/kibana/blob/main/RISK_MATRIX.mdx) |


### For maintainers

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docs/CHANGELOG.asciidoc

@@ -144,6 +144,15 @@ For information about the {kib} 8.13.0 release, review the following information
 In Canvas, an empty text element incorrectly triggers a "Markdown content is required in [readOnly] mode" toast notification. For more information, refer to ({kibana-pull}179457[#179457]).
 ====
 
+[discrete]
+[[known-177938-8.13]]
+.Index templates UI incorrectly sets the `allow_auto_create` field to `false` by default.
+[%collapsible]
+====
+*Details* +
+If you are creating or editing an index template using the Index Templates form under the Index Management page, the `allow_auto_create` field is incorrectly set to `false` by default (the default value should be `undefined`). For more information, refer to ({kibana-issue}177938[#177938]).
+====
+
 [float]
 [[breaking-changes-8.13.0]]
 === Breaking changes
@@ -532,6 +541,19 @@ Security::
 
 For information about the {kib} 8.12.0 release, review the following information.
 
+[float]
+[[known-issues-8.12.0]]
+=== Known issues
+
+[discrete]
+[[known-177938-8.12]]
+.Index templates UI incorrectly sets the `allow_auto_create` field to `false` by default.
+[%collapsible]
+====
+*Details* +
+If you are creating or editing an index template using the Index Templates form under the Index Management page, the `allow_auto_create` field is incorrectly set to `false` by default (the default value should be `undefined`). For more information, refer to ({kibana-issue}177938[#177938]).
+====
+
 [float]
 [[breaking-changes-8.12.0]]
 === Breaking changes