Drop support for Go 1.19

[FZambia]

Go 1.21 released, so it's time to bump minimal required Go version here and remove some craft.

[codecov]

Codecov Report

Merging #314 (93f3c0f) into master (d1a02db) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #314      +/-   ##
==========================================
- Coverage   84.25%   84.20%   -0.06%     
==========================================
  Files          39       38       -1     
  Lines        7439     7437       -2     
==========================================
- Hits         6268     6262       -6     
- Misses        874      877       +3     
- Partials      297      298       +1     

Files Changed	Coverage Δ
handler_http_stream.go	65.13% <100.00%> (ø)
handler_sse.go	70.62% <100.00%> (ø)

... and 1 file with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more