hooks: rename refresh hook to post-refresh

[stolowski]

It turned out after-refresh wouldn't be consistent with existing terminology (including snapcraft).

[sergiocazzolato]

Change lgtm

[zyga]

Third time's the charm

[codecov-io]

Codecov Report

Merging #3969 into master will not change coverage.
The diff coverage is 44.44%.

@@           Coverage Diff           @@
##           master    #3969   +/-   ##
=======================================
  Coverage   75.96%   75.96%           
=======================================
  Files         423      423           
  Lines       36525    36525           
=======================================
  Hits        27748    27748           
  Misses       6836     6836           
  Partials     1941     1941

Impacted Files	Coverage Δ
snap/hooktypes.go	100% <ø> (ø)	⬆️
overlord/hookstate/hookmgr.go	66.02% <100%> (ø)	⬆️
overlord/snapstate/snapstate.go	80.09% <33.33%> (ø)	⬆️
overlord/hookstate/hooks.go	15.68% <40%> (ø)	⬆️

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