Make scripted search templates work with new mediaType from XContentType.JSON

[pgomulka]

Stored scripts can have content_type option set, however when empty they default to XContentType.JSON#mediaType(). Commit 5e74f79 has changed this in master (ES8) method to return application/json;charset=utf-8 (previously application/json; charset=UTF-8)
This means that when upgrading ES from version 7 to 8 stored script will fail when being used as the encoder is being matched with string equality (map key)

elasticsearch/modules/lang-mustache/src/main/java/org/elasticsearch/script/mustache/CustomMustacheFactory.java

Lines 62 to 66 in 5e74f79

	private static final Map<String, Supplier<Encoder>> ENCODERS = Map.of(
	JSON_MIME_TYPE_WITH_CHARSET, JsonEscapeEncoder::new,
	JSON_MIME_TYPE, JsonEscapeEncoder::new,
	PLAIN_TEXT_MIME_TYPE, DefaultEncoder::new,
	X_WWW_FORM_URLENCODED_MIME_TYPE, UrlEncoder::new);

This commit address this by adding back (in addition) the old application/json; charset=UTF-8 into the encoders map.

closes #66986

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[pgomulka]

I removed the Script.java related changes to a separate PR

[jdconrad]

LGTM. Thanks for working on this!

[martijnvg]

LGTM