docs: add 3.6 json templating upgrading notes

[Joibel]

Fixes #13541
Follow up to add upgrading notes to #12909

Motivation

#12909 is a breaking change and wasn't listed in the upgrading notes

Modifications

Add upgrading notes for this change. The change is subtle and probably affects few users. Some may have worked around it as per #8930 and can optionally remove toJson from expressions, but this is not necessary.

Verification

make docs passes

[agilgur5]

While the test cases in #12909 are jsonpath specific, the code is not, as far as I understand, otherwise it could have been applied to the jsonpath helper specifically.

The wording is also duplicative and inconsistent:

Suggested change

	When accessing a structure or array from json structure in a `jsonpath` expression you could end up getting a golang representation of the structure in error.
	When accessing a map or array from JSON in a `jsonpath` expression, you would get a Golang representation.

• consistently use "JSON", not both "json" and "JSON" (esp in the same section)
  • "Golang" should similarly be capitalized as a proper noun
• simplify, per k8s style guide
  • "structure" is not needed (was previously used 3 times in one sentence) and is ambiguous, whereas "map" is a data structure consistent with "array"
  • "in error" is confusing/ambiguous as there is no error message. It's not needed either, we can just say what was returned before and what it returns now. (also whether that was concretely a mistake before is not clear either)

[Joibel]

Done, and slightly simplified again

[agilgur5]

Suggested change

	This will now return the json representation as hoped - see #12909.
	This now returns plain JSON.

• simplify, per k8s style guide

I'm also not sure we need to reference the PR and I'm not sure that referencing it would be helpful. They sometimes are and aren't referenced in the upgrading notes, but when they are it is part of the section heading and not in the contents. A header reference makes more sense to me as a way to correlate to the changelog items.

Also the PR number is not automatically linked

[Joibel]

Done

[agilgur5]

I'd probably move this near the legacy patch pod removal if the thought is that it shouldn't have much impact. I.e. order of importance (then all the metrics changes as things after it could be missed accidentally since it's a large section)

[Joibel]

I've moved it to the bottom as least important.

[isubasinghe]

LGTM