[pinpoint-apm#2781] pinpoint 1.6.1 release

version change (1.6.1-SNAPSHOT -> 1.6.1)

[pinpoint-apm#2767] 1.6.1-RC2 release

(1.6.1-SNAPSHOT -> 1.6.1-RC2)

[pinpoint-apm#2708] 1.6.1-RC1 release

(1.6.1-SNAPSHOT -> 1.6.1-RC1)
(cherry picked from commit fee31af)

Add a note on 1.6.0

Merge pull request pinpoint-apm#1671 from koo-taejin/pinpoint-apm#1668

1.5.2 release pinpoint-apm#1668

1.5.1 release

1.5.0 tagging

1.1.2 tagging