net/s3m: update to 0.13.1

While here:

• remove MASTER_SITES and DISTFILES Source is available at GitHub and specifying both MASTER_SITES and USE_GITHUB is redundant.
• remove GH_ACCOUNT which is defaulted to PORTNAME

Changelog: https://github.com/s3m/s3m/blob/0.13.1/CHANGELOG.md

PR: 287539
Reported by: Nicolas Embriz <nbari@tequila.io> (maintainer)