lang/go126: Update to 1.26.4
Changes:
go1.26.4 (released 2026-06-02) includes security fixes to the crypto/x509, mime, and net/textproto packages, as well as bug fixes to the compiler, the runtime, the go fix command, and the crypto/fips140 package.
MFH: 2026Q2
Reviewed by: adamw
Differential Revision: https://reviews.freebsd.org/D57407