Modernize dependencies and bump Go version to 1.21

[kiro-agent]

This pull request was generated by @kiro-agent 👻

Comment with /kiro fix to address specific feedback or /kiro all to address everything.
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Summary

This PR modernizes the project's dependencies and Go version:

• Remove github.com/pkg/errors — replaced all usages with Go's built-in fmt.Errorf("...: %w", err) error wrapping (available since Go 1.13). This eliminates the only external non-x/ dependency.
• Upgrade golang.org/x/time from v0.0.0-20200630173020 (a 2020 pseudo-version) to v0.10.0
• Replace deprecated ioutil.ReadAll with io.ReadAll (available since Go 1.16)
• Bump go directive from 1.13 to 1.21

Changes across 8 files

File	Change
go.mod / go.sum	Removed pkg/errors, upgraded x/time, bumped Go version
card.go	errors.Wrapf/Errorf → fmt.Errorf with %w
client.go	errors.Wrapf → fmt.Errorf with %w; ioutil.ReadAll → io.ReadAll
webhook.go	errors.Wrapf → fmt.Errorf with %w
trello.go	errors.Wrapf → fmt.Errorf with %w
list-duration.go	errors.Wrap → fmt.Errorf with %w
member-duration.go	errors.Wrap → fmt.Errorf with %w

Testing

All existing tests pass (go test ./...).