[DISCUSS] Use "backward-incompatible" label in JIRA

[Stamatis Zampetakis <za...@gmail.com>]

Hi all,

In order to track changes who change the behavior of an existing component
or break public APIs we could adopt the use of JIRA labels as it is done in
other projects.

There are various existing labels used for this purpose in JIRA:
* Breaking-Change,
* breaking-api,
* breaking,
* breaking_change,
* backwards-incompatible,
* backward-incompatible

The last one, "backward-incompatible", seems to be the most widely used
across projects and in Hive itself so I would suggest sticking to that one
and using it consistently.

I just updated existing Hive tickets to use "backward-incompatible" and
removed the rest mentioned above. I also added a brief mention to the
existing guidelines [1].

If you prefer another one that's perfectly fine as well.

Best,
Stamatis

[1]
https://cwiki.apache.org/confluence/display/Hive/HowToContribute#HowToContribute-Guidelines