rename membercluster api to cluster

[kevin-wangzefeng]

Signed-off-by: Kevin Wang kevinwzf0126@gmail.comWhat type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind api-change

/kind bug
/kind cleanup
/kind deprecation
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
To simplify API group and name:

• change apiGroup from membercluster.karmada.io to cluster.karmada.io
• change kind from membercluster to cluster

Which issue(s) this PR fixes:
Fixes #
Part of #140
Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[RainbowMango]

Thanks for your hard work!