net/syncthing: Fix broken build and upgrade to 1.20.1

After lang/go was updated to 1.18, net/syncthing failed to build under
poudriere, with errors similar to:

vendor/github.com/marten-seemann/qtls-go1-18/cipher_suites.go:143:43: undeclared name: any (requires version go1.18 or later)

This is because the packaged vendor libraries in the upstream tarball
don't work with go 1.18. Fix this by using USES=go:modules and changing
the build commands. Also, upgrade to 1.20.1.

PR: 262808
Reported by: Oscar Carlsson <oscar@spindel.tax>
Approved by: maintainer timeout (2 weeks)
Submitted by: dmgk
MFH: 2022Q2