Installation docs need to specify correct version of GTDB to download (max R214)

[smdabdoub]

Since Aviary installs GTDB-tk v2.3.2, users can download, at most, GTDB R214.

GTDB-tk v2.4.0 switches from fastani to skani which changes paths in the database causing the GTDB task in Aviary to fail with the following message:

ERROR: Reference genome missing from FastANI database: release220/fastani/database/GCF...