Set version number to 0.21.0rc1 for first release candidate

[mtreinish]

Summary

For the 0.21.0 release we're going to start pushing release candidates
prior to the release to enable testing before we cut the final release.
In preparation for tagging the first release candidate this commit
updates the version string to indicate it's a release candidate.

Details and comments

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[mtreinish]

I debated doing the release notes for this PR, but we can wait for that until we tag the real release since we're not publishing the rc1 release notes (or at least I wasn't planning on it because we don't bump the metapackage).

[coveralls]

Pull Request Test Coverage Report for Build 2543447384

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 84.289%

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Totals
Change from base Build 2542722489:	0.001%
Covered Lines:	54971
Relevant Lines:	65217

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💛 - Coveralls

[jakelishman]

In principle, this looks fine to me. Assuming all the issues with the IBMQ provider are fixed now, this looks good to merge.

A minor unrelated question: does the Cargo.lock file get automatically updated (on build) only because the Cargo.toml changed, or is it just that we haven't been committing other changes that might have occurred?

[mtreinish]

A minor unrelated question: does the Cargo.lock file get automatically updated (on build) only because the Cargo.toml changed, or is it just that we haven't been committing other changes that might have occurred?

That's on me. I had run cargo update locally to bump things as part of an MSRV test and I committed it when I bumped the version string in the Cargo.toml. It was a conscious decision on my part I figure get everything up to date since it works with the new msrv prior to the release. (but I can revert it if you'd prefer)

Typically the Cargo.lock won't be updated unless you change something in the Cargo.toml and rebuild (and then it will only be related to that change) or you manually update it. So in this case if I hadn't run cargo update the only change to the Cargo.lock would be the qiskit-terra crate version bump: https://github.com/Qiskit/qiskit-terra/pull/8200/files#diff-13ee4b2252c9e516a0547f2891aa2105c3ca71c6d7a1e682c69be97998dfc87eR344