Rename devel/oniguruma6 to devel/oniguruma

Details:

• Previous, unmaintained versions of devel/oniguruma have been cleared from the ports tree.
• No ports depend on outdated versions of devel/oniguruma.
• Only one version is supported upstream.

PR: 222869
Reported by: m.bueker@berlin.de
Approved by: maintainer timeout