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

[leighmcculloch]

Now that we've separated the protocol and pre-release version in:

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

If we wanted to remove the pre-release version in the future we could do so by simply stopping using it, and renaming the field from preRelease to reserved or deprecated, just expecting it to be forever zero.