Remove unnecessary build tag go1.21

[alexandear]

This PR reverts changes from #1695 because they are unnecessary.

Short Explanation

According to the Go documentation on Build Constraints, the constraint go1.20 applies to go1.20, go1.21, go1.22, and so on. Conversely, !go1.20 applies to go1.18 and go1.19.

Here's the relevant excerpt from the documentation:

During a particular build, the following build tags are satisfied:
...

• a term for each Go major release, through the current version: "go1.1" from Go version 1.1 onward, "go1.12" from Go 1.12, and so on.

Long Explanation

Currently, fasthttp supports the following Go versions: 1.18, 1.19, 1.20, 1.21, 1.22.
With build tags, we should ensure that:

• s2b_new.go is included only when building with Go 1.20, 1.21, 1.22;
• s2b_old.go is used for Go versions 1.18 and 1.19.

Let's implement the changes in this PR and list the files used for building with different Go versions.

Go 1.18

This version compiles s2b_old.go:

go version go1.18 darwin/arm64
❯ go1.18 list -f '{{.GoFiles}}' ./...
[args.go b2s_old.go brotli.go bytesconv.go bytesconv_64.go bytesconv_table.go client.go coarsetime.go compress.go cookie.go doc.go fs.go header.go headers.go http.go lbclient.go methods.go nocopy.go peripconn.go round2_64.go s2b_old.go server.go status.go stream.go streaming.go strings.go tcp.go tcpdialer.go timer.go tls.go uri.go uri_unix.go userdata.go workerpool.go]
...

Right.

Go 1.19

This version compiles s2b_old.go:

❯ go1.19 version
go version go1.19 darwin/arm64
❯ go1.19 list -f '{{.GoFiles}}' ./...
[args.go b2s_old.go brotli.go bytesconv.go bytesconv_64.go bytesconv_table.go client.go coarsetime.go compress.go cookie.go doc.go fs.go header.go headers.go http.go lbclient.go methods.go nocopy.go peripconn.go round2_64.go s2b_old.go server.go status.go stream.go streaming.go strings.go tcp.go tcpdialer.go timer.go tls.go uri.go uri_unix.go userdata.go workerpool.go]
...

Right.

Go 1.20

This version compiles s2b_new.go:

❯ go1.20 version
go version go1.20 darwin/arm64
❯ go1.20 list -f '{{.GoFiles}}' ./...
[args.go b2s_new.go brotli.go bytesconv.go bytesconv_64.go bytesconv_table.go client.go coarsetime.go compress.go cookie.go doc.go fs.go header.go headers.go http.go lbclient.go methods.go nocopy.go peripconn.go round2_64.go s2b_new.go server.go status.go stream.go streaming.go strings.go tcp.go tcpdialer.go timer.go tls.go uri.go uri_unix.go userdata.go workerpool.go]
...

Yeah, this is what we want.

Go 1.21

This version compiles s2b_new.go:

❯ go1.21.7 version
go version go1.21.7 darwin/arm64
❯ go1.21.7 list -f '{{.GoFiles}}' ./...
[args.go b2s_new.go brotli.go bytesconv.go bytesconv_64.go bytesconv_table.go client.go coarsetime.go compress.go cookie.go doc.go fs.go header.go headers.go http.go lbclient.go methods.go nocopy.go peripconn.go round2_64.go s2b_new.go server.go status.go stream.go streaming.go strings.go tcp.go tcpdialer.go timer.go tls.go uri.go uri_unix.go userdata.go workerpool.go]
...

Right.

Go 1.22

This version compiles s2b_new.go:

❯ go version
go version go1.22.0 darwin/arm64
❯ go list -f '{{.GoFiles}}' ./...
[args.go b2s_new.go brotli.go bytesconv.go bytesconv_64.go bytesconv_table.go client.go coarsetime.go compress.go cookie.go doc.go fs.go header.go headers.go http.go lbclient.go methods.go nocopy.go peripconn.go round2_64.go s2b_new.go server.go status.go stream.go streaming.go strings.go tcp.go tcpdialer.go timer.go tls.go uri.go uri_unix.go userdata.go workerpool.go]
...

Cool.

[alexandear]

Looks like we have flaky tests.

[erikdubbelboer]

Thanks!