fix: Address modernize.omitzero issues

[alexandear]

This PR enables the previously disabled modernize.omitzero check.

Fixed issues

1. markReadOptions.LastReadAt:

We can safely change omitzero because it's an unexported struct. Now we can pass an empty Timestamp{} to MarkNotificationsRead and MarkRepositoryNotificationsRead, which means marking all notifications as read.

2. RepositoryContentResponse.Commit:

It's a required field according to https://docs.github.com/en/rest/repos/contents?apiVersion=2022-11-28#create-or-update-file-contents.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.53%. Comparing base (521b62a) to head (e716b1d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3972   +/-   ##
=======================================
  Coverage   93.53%   93.53%           
=======================================
  Files         207      207           
  Lines       17586    17586           
=======================================
  Hits        16449    16449           
  Misses        937      937           
  Partials      200      200           

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[gmlewis]

Thank you, @alexandear!
LGTM.

As far as I can tell, though, this PR does NOT break the existing API, and therefore the PR title can change back to "fix:" instead of "fix!:". Do you agree?

Awaiting second LGTM+Approval from any other contributor to this repo before merging.

cc: @stevehipwell - @zyfy29 - @Not-Dhananjay-Mishra

[stevehipwell]

If you're updating the code for omitzero, the environment created update struct had annotations removed before omitzero was available. The omitzero annotation could now be added to reviewers and one of the other fields (can't remember the name off the top of my head).

[Not-Dhananjay-Mishra]

LGTM

[gmlewis]

Thank you, @alexandear, @stevehipwell, and @Not-Dhananjay-Mishra!
Merging.