Introduce v3.3.0-rc6 before Gerrit v3.3 release

The recent changes in the stable-3.3 (see [1]) and the P0 bug
associated with the JGit autoGc (see [2]) have pushed the v3.3
release date by one more week, for allowing more testing before
the final release.

[1] https://gerrit-review.googlesource.com/q/hashtag:attentionset-3.3
[2] https://bugs.chromium.org/p/gerrit/issues/detail?id=13544

Change-Id: I493e5d2246b417681a770a262c614ae4196abe86
1 file changed