fix: use octokit.rest interface for github

[erikburt]

I broke this with the node20 upgrade in #31. The octokit.pulls, octokit.checks objects are now nested inside octokit.rest, which broke existing usages. This was not uncovered with unit tests because the jest mocking covered this.

This PR:

1. switches all github calls to use octokit.rest
2. updates the unit tests accordingly

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Testing

Tested locally this time by running things like:

DEBUG="true" INPUT_TOKEN="github_pat_<pat>" GITHUB_REPOSITORY="adRise/update-pr-branch" INPUT_REQUIRE_AUTO_MERGE_ENABLED="false" INPUT_REQUIRE_PASSED_CHECKS="false" INPUT_BASE="master" INPUT_REQUIRED_APPROVAL_COUNT="0" node dest/index.js 

Example of errors before this fix:

TypeError: Cannot read properties of undefined (reading 'list')
TypeError: Cannot read properties of undefined (reading 'updateBranch') 
TypeError: Cannot read properties of undefined (reading 'checks')

[erikburt]

@zhiyelee Sorry for breaking it.

[coveralls]

Pull Request Test Coverage Report for Build 9274075280

Details

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 9249736164:	0.0%
Covered Lines:	98
Relevant Lines:	98

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💛 - Coveralls

[zhiyelee]

@erikburt Thanks for the report and fix. No worries. Will take a look and release a new patch release soon