Remove pre_release_version from meta.rs

[sisuresh]

What problem does your feature solve?

pre_release_version was helpful for the development of protocol 20 because of how often we iterated on the protocol, but going forward, we won't breaki a non-vnext build intentionally. The interface version (which contains both the protocol version and pre-release version) has caused confusion in the past, so we should just remove it.

[leighmcculloch]

I propose an alternative solution to the problem in:

• Replace the Env Meta Interface Version with two new meta fields for the Protocol Version and the Pre-Release Version stellar-protocol#1476