hs-v3: Change "hsdir-interval" to "hsdir_interval" to match the spec
We've merged the consensus param used by HS v3 in dir-spec.txt (#24118 (moved)). They all follow some sort of common naming convention using underscores instead of hyphens.
The hsdir-interval is the only one in the code not following this. It is in get_time_period_length()
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