fix(jwt): set issued_at to nil prior to validating claims

[ecrupper]

Due to some clock skew, we began seeing token used before issued errors. We noticed this issue in the JWT library, which appears to be resolved in their upcoming v5 release. In that release, they will be making issued_at unchecked by default, due to its optional nature. I elaborated on this in the code comments as well.

[codecov]

Codecov Report

Merging #813 (9cb91a5) into v0.18.2 (a3ab6f0) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 9cb91a5 differs from pull request most recent head bfe9428. Consider uploading reports for the commit bfe9428 to get more accurate results

@@             Coverage Diff             @@
##           v0.18.2     #813      +/-   ##
===========================================
+ Coverage    54.87%   54.88%   +0.01%     
===========================================
  Files          244      244              
  Lines        16506    16510       +4     
===========================================
+ Hits          9058     9062       +4     
  Misses        7048     7048              
  Partials       400      400              

Impacted Files	Coverage Δ
internal/token/parse.go	91.89% <100.00%> (+0.98%)	⬆️