lang/go126: Update to 1.26.3
Changes:
go1.26.3 (released 2026-05-07) includes security fixes to the go command, the pack tool, and the html/template, net, net/http, net/http/httputil, net/mail, and syscall packages, as well as bug fixes to the go command, the go fix command, the compiler, the linker, the runtime, and the crypto/fips140, crypto/tls, go/types, and os packages.
PR: 295109