[fern-autoversion] feat!: migrate module path from /v2 to unversioned root

The Go module path has been updated from `github.com/VapiAI/server-sdk-go/v2`
to `github.com/VapiAI/server-sdk-go` across all packages. This is a breaking
change for all existing consumers: every import path in user code must be
updated to drop the `/v2` suffix.

Additionally, the `User-Agent` header previously set to
`github.com/VapiAI/server-sdk-go/v2.0.0` has been removed from
`core/request_option.go`, and `.fernignore` has been updated to exclude
replay and gitattributes files.

Key changes:
- All internal import paths changed from `github.com/VapiAI/server-sdk-go/v2/...` to `github.com/VapiAI/server-sdk-go/...`
- `User-Agent` header removed from `RequestOptions.cloneHeader()`
- `.fernignore` updated to exclude `.fern/replay.lock`, `.fern/replay.yml`, and `.gitattributes`

🌿 Generated with Fern

Author: fern-api[bot]

.fern/metadata.json

@@ -6,5 +6,5 @@
     "union": "v1"
   },
   "originGitCommit": "2c66caa46a7889ac44f77d16914eb96a3641e6af",
-  "sdkVersion": "v0.0.0-fern-placeholder"
+  "sdkVersion": "v3.0.0"
 }