all: drop Go-1.19, add Go-1.21

[codecov[bot]]

Codecov Report

Patch coverage: 71.42% and project coverage change: -0.06% :warning:

Comparison is base (bd7e871) 67.63% compared to head (e16dc3e) 67.57%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #144 +/- ## ========================================== - Coverage 67.63% 67.57% -0.06% ========================================== Files 29 29 Lines 2555 2560 +5 ========================================== + Hits 1728 1730 +2 - Misses 724 727 +3 Partials 103 103 ``` | [Files Changed](https://app.codecov.io/gh/go-zeromq/zmq4/pull/144?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=go-zeromq) | Coverage Δ | | |---|---|---| | [rep.go](https://app.codecov.io/gh/go-zeromq/zmq4/pull/144?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=go-zeromq#diff-cmVwLmdv) | `69.43% <71.42%> (-0.26%)` | :arrow_down: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/go-zeromq/zmq4/pull/144/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=go-zeromq)

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