Update release instructions for 4.6-maintenance

[asottile]

I'm going to run these and make sure they make sense

[codecov]

Codecov Report

Merging #5363 into master will decrease coverage by 0.47%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5363      +/-   ##
==========================================
- Coverage   93.43%   92.96%   -0.48%     
==========================================
  Files         115      115              
  Lines       26399    26399              
  Branches     2606     2606              
==========================================
- Hits        24667    24542     -125     
- Misses       1409     1530     +121     
- Partials      323      327       +4

Impacted Files	Coverage Δ
testing/python/collect.py	68.92% <0%> (-16.28%)	⬇️
testing/test_pytester.py	82.81% <0%> (-6.48%)	⬇️
testing/python/metafunc.py	82.06% <0%> (-4.74%)	⬇️
testing/test_pluginmanager.py	49.57% <0%> (-1.28%)	⬇️
testing/python/fixtures.py	90.12% <0%> (-1.25%)	⬇️
src/_pytest/python.py	91.63% <0%> (-0.49%)	⬇️
testing/test_warnings.py	60.65% <0%> (+11.47%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 733f43b...4aec893. Read the comment docs.

[asottile]

updated the instructions and used them to create #5365