This updates `ElectronBuilder` to use `Yaml#find_versions`, as the
only code unique to that strategy is to restrict regex usage and
use default version-finding logic when a `strategy` block isn't
provided. This is similar to how we have various strategies that
use `PageMatch#find_versions` internally.
This allows us to remove `ElectronBuilder#versions_from_content`
entirely, along with the related tests. I've added support for
`provided_content` to `ElectronBuilder#find_versions` as a way of
adding tests and maintaining code coverage.
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