fix(docs): correct go install instructions

[jdahm]

Go 1.21+ is required for recent versions of sqlc. This is because it uses components that require the 1.21 toolchain. Coincidentally 1.21 is also when forward compatibility was introduced.

You can test this

$ docker run -it --rm golang:1.19
% go install github.com/sqlc-dev/sqlc/cmd/sqlc@latest
...
pkg/mod/github.com/sqlc-dev/sqlc@latest/internal/bundler/upload.go:7:2: package log/slog is not in GOROOT (/usr/local/go/src/log/slog)
pkg/mod/github.com/sqlc-dev/sqlc@latest/internal/codegen/golang/opts/options.go:6:2: package maps is not in GOROOT (/usr/local/go/src/maps)

[jdahm]

I removed this since it's irrelevant now that Go 1.21 is required for maps and slog.