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from this page is outdated and may suffer from known security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