unmaintained/seamonkey: move from testing

no bumps for upstream security updates since 2.53.1 was released Feb 2020. note [0] "Warning! The SeaMonkey version available for download from this page is outdated and may suffer from known security vulnerabilities." for this type of highly exposed security-sensitive software, there must be a maintainer responsible for regular and timely updates, not just once every couple years.

[0] https://www.seamonkey-project.org/releases/2.49.5

i can bump it this one time, but am not interested in undertaking that responsibility for future versions. if there is nobody else that wants to, then it should be moved to unmaintained.

closes #11242 (closed).