chore: running some chores for notation-go

[Two-Hearts]

In this PR:

1. According to issue Update references from Notary v2 to Notation #287, updated Notary v2 to Notation.
2. Updated error messages of trust policy to improve user experience. Related issue: Improve the error for missing trust policy notation#621

Resolves #287

[codecov-commenter]

Codecov Report

Merging #311 (c508019) into main (1b9b7f2) will not change coverage.
The diff coverage is 66.66%.

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@@           Coverage Diff           @@
##             main     #311   +/-   ##
=======================================
  Coverage   74.39%   74.39%           
=======================================
  Files          23       23           
  Lines        2175     2175           
=======================================
  Hits         1618     1618           
  Misses        439      439           
  Partials      118      118           

Impacted Files	Coverage Δ
plugin/proto/verify.go	0.00% <ø> (ø)
verifier/trustpolicy/trustpolicy.go	92.24% <66.66%> (ø)

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[yizha1]

Thanks @Two-Hearts. I left a few comments

[shizhMSFT]

LGTM

[priteshbandi]

LGTM