Update to re2 2.0.0

What does this MR do and why?

There are no code changes between 2.0.0 and 2.0.0.beta2 (https://my.diffend.io/gems/re2/2.0.0.beta2/2.0.0), but let's use a released version instead of a pre-release. This commit also relaxes the version constraint since the pre-release version must be specified explicitly.

See: https://github.com/mudge/re2/releases/tag/v2.0.0

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.