Plan to move td-agent 3 to maintenance mode

[repeatedly]

We have released td-agent 4 last year and some users use it on production without any issues.
td-agent 4 uses newer ruby/components and use more smarter packaging approach. No reason to use td-agent 3 for new deployment.
Time to move td-agent 3 to maintenance mode.

See also: #219

[ashie]

td-agent 3 uses Ruby 2.4 which isn't supported anymore, so we should announce dropping schedule of td-agent 3.

Past announce: https://www.fluentd.org/blog/drop-schedule-announcement-in-2019