Update Dataflow Flex template README (#8149)
Update Dataflow Flex template README (#8149)

Replace the list of Flex Templates limitations (which is out-of-date) with the link to the Dataflow documentation page on Flex Templates which is the authoritative source for the up-to-date list of limitations.

Co-authored-by: Dan Lee <73a4690aaee1868a8cc9d982a8fc87c8f9121c2e@users.noreply.github.com>
GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples