MRG: minor updates to release procedure after 4.8.4

[ctb]

This PR:

• changes git checkout -b to git switch -c
• removes unused testenv4
• adds checklist items for spack and pyodide
• adds more info on version bump to v+1-dev

Fixes #2517
Addresses #2718 (comment)

[ctb]

Ready for review @sourmash-bio/devs

[codecov]

Codecov Report

Merging #2783 (da4854b) into latest (966c045) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           latest    #2783   +/-   ##
=======================================
  Coverage   85.94%   85.94%           
=======================================
  Files         130      130           
  Lines       14808    14808           
  Branches     2621     2621           
=======================================
  Hits        12727    12727           
  Misses       1780     1780           
  Partials      301      301           

Flag	Coverage Δ
hypothesis-py	25.82% <ø> (ø)
python	92.85% <ø> (ø)
rust	49.93% <ø> (ø)

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[bluegenes]

lgtm