certify availability using FLAC during correct time period

[Jacob-Stevens-Haas]

See the wiki:

We have recently switched our primary storage format for Audio Data from compressed wav files to FLAC files. FLAC is more space and CPU efficient, and is widely supported. As of April 9, 2021, all hydrophones produce FLAC files as source. Prior to that, there was a transition period when FLAC files were generated from the source wav files and archived as well, that period was August 10, 2020 to April 9, 2021. During this transition period, both wav and FLAC formats were archived. File formats that are archived are readily available, while other formats are generated on-the-fly and are much slower to access. For fast results for lossless audio data, select wav file formats up to August 10, 2020, and select FLAC thereafter. The data availability graph in Data Search maybe updated in the future to show which files are available directly, while the archivefiles service can offer a list of files in the archive as well.

since tehom.downloads.certify_acoustic_availability searches the archive for wav files to determine what data range is available, it will see all data as stopped after April 9 2021.