Support x-ratelimit-reset handling for secondary rate limits

[gofri]

GitHub API states that x-ratelimit-reset might be returned instead of the Retry-After header for secondary rate limits.
Seems like GitHub recently started using it. Some evidence:

• Callback function doesn't trigger after secondary rate limit is hit gofri/go-github-ratelimit#9
• https://github.com/orgs/community/discussions/50326

This PR handles this case as a fallback for Retry-After, and cleans the code a bit while at it

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[codecov]

Codecov Report

Merging #2775 (e33c536) into master (a938017) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #2775 +/- ##
=======================================
Coverage 98.05% 98.05% =======================================
Files 132 132 Lines 11638 11648 +10 =======================================
+ Hits 11412 11422 +10 
Misses 154 154 Partials 72 72 

Impacted Files	Coverage Δ
github/github.go	97.97% <100.00%> (+0.02%)	⬆️

[gmlewis]

Thank you, @gofri !
LGTM.

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

[gmlewis]

Thank you, @valbeat !
Merging.