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

Remove the `/v2` suffix from all Go module import paths across the entire
SDK, migrating from `github.com/VapiAI/server-sdk-go/v2/...` to
`github.com/VapiAI/server-sdk-go/...`. This is a breaking change for all
existing consumers who must update their import paths accordingly.

Also removes the `User-Agent` header (`github.com/VapiAI/server-sdk-go/v2.0.0`)
from outgoing requests and adds `.fern/replay.lock`, `.fern/replay.yml`,
and `.gitattributes` to `.fernignore`.

Key changes:
- All internal and sub-package imports updated from `.../v2/...` to `.../...`
- `User-Agent` header removed from `core/request_option.go`
- `.fernignore` updated to exclude Fern replay and git attribute files

🌿 Generated with Fern

Author: fern-api[bot]

.fern/metadata.json

@@ -6,5 +6,5 @@
     "union": "v1"
   },
   "originGitCommit": "c93e6036883761900b393007e64caf65f39cf782",
-  "sdkVersion": "v0.0.0-fern-placeholder"
+  "sdkVersion": "v3.0.0"
 }