Update fastly/fastly requirement from ^11.0 to ^12.1

Updates the requirements on [fastly/fastly](https://github.com/fastly/fastly-php) to permit the latest version.
- [Release notes](https://github.com/fastly/fastly-php/releases)
- [Changelog](https://github.com/fastly/fastly-php/blob/main/CHANGELOG.md)
- [Commits](fastly/fastly-php@v11.0.0...v12.1.0)

---
updated-dependencies:
- dependency-name: fastly/fastly
  dependency-version: 12.1.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

composer.json

@@ -10,7 +10,7 @@
     ],
     "require": {
         "php": "^8.1",
-        "fastly/fastly": "^11.0",
+        "fastly/fastly": "^12.1",
         "guzzlehttp/guzzle": "^7.5",
         "ray/di": "^2.13.1"
     },