docs: Update docs that still use deprecated transforms

[pront]

Example: https://vector.dev/guides/level-up/managing-schemas/#example-filtering-data-for-gdpr-compliance

From this:

[transforms.parse]
inputs = ["application"]
drop_field = true
drop_invalid = false
type = "json_parser"

To this:

[transforms.parse]
inputs = ["application"]
type = "remap"
source = '''
. = parse_json!(.message)
'''

Other occurrences: https://github.com/search?q=org%3Avectordotdev+%22type+%3D+%5C%22json_parser%5C%22%22&type=code