docs(CHANGELOG.md): add dependency bump from dependabot

Author: dependabot[bot]

CHANGELOG.md

@@ -18,6 +18,7 @@
 - build(deps): `golangci/golangci-lint-action` from 8 to 9 ([#767](https://github.com/fastly/go-fastly/pull/767))
 - build(deps): `golang.org/x/crypto` from 0.43.0 to 0.44.0 ([#768](https://github.com/fastly/go-fastly/pull/768))
 - build(deps): `golang.org/x/crypto` from 0.44.0 to 0.45.0 ([#769](https://github.com/fastly/go-fastly/pull/769))
+- build(deps): `actions/checkout` from 5 to 6 ([#770](https://github.com/fastly/go-fastly/pull/770))
 
 ## [v12.1.0](https://github.com/fastly/go-fastly/releases/tag/v12.1.0) (2025-09-30)