databases/mysql55-server: Mark DEPRECATED

According to the upstream document
https://www.oracle.com/us/support/library/lifetime-support-technology-069183.pdf
MySQL 5.5 has reached it's Extended Support on December 2018. Under
sustaining support there is no New updates, fixes, security alerts, data
fixes, and critical patch updates. Hence we are setting it to EXPIRE
before 2022Q3.

PR:		263762
Approved by:	ale (maintainer)