fix(build): fix NBGV version propagation for NuGet and Docker

- Delete src/version.json (stale at 0.13, was overriding root version.json 0.15)
- Add tag ref pattern to publicReleaseRefSpec so tag-triggered builds produce
  clean release versions without prerelease suffixes

Root cause: NBGV walks up from the project directory and finds the closest
version.json. The src/version.json (0.13) was found before root version.json
(0.15), causing all library builds to use version 0.13.x.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: csharpfritz

src/version.json

@@ -4,7 +4,8 @@
   "publicReleaseRefSpec": [
     "^refs/heads/master$",
     "^refs/heads/main$",
-    "^refs/heads/v\\d+(?:\\.\\d+)?$"
+    "^refs/heads/v\\d+(?:\\.\\d+)?$",
+    "^refs/tags/v\\d+\\.\\d+(\\.\\d+)?$"
   ],
   "cloudBuild": {
     "buildNumber": {