Support a wider array of version go files

My change request is here: https://github.com/googleapis/release-please/pull/1891

Basically, I want to support `version.go` files like this: https://github.com/taikoxyz/taiko-client/blob/main/version/version.go, but when I looked at the code for parsing the `version.go` file I noticed it's strict on declaring `Version` as `const`.

Can we make this more flexible, or, is there a good reason not to? Second of all, does the `version.go` file need to be in the root of the monorepo to get bumped? I am trying to make it work for my project: https://github.com/taikoxyz/taiko-client

Thank you!