Bump pallet-contracts version to 3.0.0-rc1

[athei]

This bump is performed in expectation of a new substrate release soon(TM). There are already breaking changes in master [1, 2] and more are expected [1, 2] before 3.0.0 can be released.

The rcX suffix is a workaround to not be forced to bump the major version in rapid succession in this phase of applying breaking changes before we go over to a more stable period (after 3.0.0 release).

@Robbepop @seanyoung

[athei]

After talking to @gnunicorn I realized that this workaround is not necessary. By simply not bumping the version of pallet-contracts we can prevent the release of a new version during the next release cycle. This is what I prefer to do until we are ready to release 3.0.0-rcX.