Change version number to 0.25-preview

Our new version numbering system is quite clever and tries to advance
the teeny version number automagically.  Instead of versioning
prereleases as `0.25.x` where `x` is monotonically increasing, version
our prereleases as `0.25-preview`.

This is roughly what our previous strategy looked like.

version.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://raw.githubusercontent.com/AArnott/Nerdbank.GitVersioning/master/src/NerdBank.GitVersioning/version.schema.json",
-  "version": "0.25",
+  "version": "0.25.0-preview.{height}",
   "publicReleaseRefSpec": [
     "^refs/heads/master$", // we release out of master
     "^refs/heads/v\\d+(?:\\.\\d+)?$" // we also release out of vNN branches