Consolidate preamble comment warnings

[a-frantz]

See an example of a "bad" preamble here: https://github.com/aws-samples/amazon-omics-tutorials/blob/main/example-workflows/gatk-best-practices/workflows/germline-fastqs-to-vcf/sub-workflows/fastq-to-ubam.wdl

I'm not sure who owns this code (whether it's the Broad or Amazon), but they are using double-pound sign comments the way we use them, but after the version declaration. This causes a LintWarning to be thrown for every line of "the preamble" of that file. We should update to detect a continuous preamble in the wrong location and warn once on the whole span.