Fix indices.get_template doc link and deprecation

[swallez]

Reported in https://discuss.elastic.co/t/templates-w-new-java-client/300218

The index.get_template endpoint is deprecated and has been replaced by index.get_index_template since 7.8.

The docs URL in json-spec points to the non-deprecated version (correct version here) and has no deprecation indication.

The upstream json-spec should be fixed and this issue serves to track the update of schema.json.