Add dandi-staging to known_instances

[dchiquito]

There is now a staging API server set up at https://api-staging.dandiarchive.org, so it would be cool if dandi-cli could upload to it.

[codecov]

Codecov Report

Merging #621 (86e9b1e) into master (0dabaf4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #621   +/-   ##
=======================================
  Coverage   83.89%   83.89%           
=======================================
  Files          59       59           
  Lines        5701     5701           
=======================================
  Hits         4783     4783           
  Misses        918      918           

Flag	Coverage Δ
unittests	83.89% <ø> (ø)

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Impacted Files	Coverage Δ
dandi/consts.py	100.00% <ø> (ø)

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[yarikoptic]

sweet, didn't try it but let's proceed.

BTW, we left

    "dandi-devel": dandi_instance(
        "https://gui-beta-dandiarchive-org.netlify.app",
        None,
        None,
    ),

in place -- is that web ui instance ever to come back or should we kill it?