Simple strategy does not allow v prefix in version files

The `simple` strategy just overwrites the contents of `version-file` with `X.Y.Z`. This is a problem for my usecase because I need this file to contain `v.X.Y.Z`.

I can't use the `extra-files`method discussed in some other issues because I rely on this file only containing the version string, putting the `x-release-please-version` flag in my version.txt is going to break many places where i `cat` this file and expect only the version in the content.

I'm proposing that if the version file contains a version already, then we just update the version in place like we do for `x-release-please-version`, rather than overwrite the file. I've implemented this solution in https://github.com/googleapis/release-please/pull/2537