Docs: update install instructions for Go 1.17+ and mention distro packages

[sauryagur]

The current install docs still reference go get and GOPATH workflows, which have been deprecated since Go 1.17. Following them on a modern Go toolchain results in errors like:

go: -d flag is deprecated. -d=true is a no-op
go: go.mod file not found in current directory or any parent directory.
	'go get' is no longer supported outside a module.
	To build and install a command, use 'go install' with a version,
	like 'go install example.com/cmd@latest'
	For more information, see https://golang.org/doc/go-get-install-deprecation
	or run 'go help get' or 'go help install'.

The recommended install is:

# Latest version with postgres support
go install -tags 'postgres' github.com/golang-migrate/migrate/v4/cmd/migrate@latest

# Or pin a version
go install -tags 'postgres' github.com/golang-migrate/migrate/v4/cmd/migrate@v4.17.0

Additionally, Fedora users can install migrate via the package manager:

sudo dnf install migrate

Proposal:

• Replace go get instructions with go install examples (Go ≥1.17).
• Add a small “Linux distro packages” section mentioning Fedora’s dnf package and leaving room for others (brew, apt, etc.).

Happy to open a PR with these changes if this direction makes sense.