Allow the creation of prereleases above `v1.0`

Thanks for stopping by to let us know something could be better!

**Is your feature request related to a problem? Please describe.**

Currently it seems creating prerelease versions above `1.0` are not allowed. I believe [this line](https://github.com/googleapis/release-please/blob/ace2bd5dc778f83c33ad5dee6807db5d0afdba36/src/manifest.ts#L1160) is responsible for preventing that. I'm curious if this is intended or if the maintainers would be willing to allow this behavior.

My workflow creates a prerelease for integration testing, for which a image is built and pushed to that environment. Once those tests are successful we promote it by changing the GitHub prerelease to a full release. This process worked fine until a repo reached `v1.0.0` after which I observed Release Please was no longer creating prereleases and pipelines started breaking.

**Describe the solution you'd like**

Provide a way for Release Please to respect `prerelease: true` irrespective of the released version.

**Describe alternatives you've considered**

I have considered a `prerelease` versioning strategy, but that would require extra effort to add and remove versioning strings or relabel container images. That very well might work but is less clean (in my opinion) than what was previously working just fine.

**Additional context**

None.