add slsa l1 badge (flutter#125932)

Adds SLSA L1 badge

README.md

@@ -13,6 +13,7 @@
 [![codecov](https://codecov.io/gh/flutter/flutter/branch/master/graph/badge.svg?token=11yDrJU2M2)](https://codecov.io/gh/flutter/flutter)
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/5631/badge)](https://bestpractices.coreinfrastructure.org/projects/5631)
 [![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/flutter/flutter/badge)](https://deps.dev/project/github/flutter%2Fflutter)
+[![SLSA 1](https://slsa.dev/images/gh-badge-level1.svg)](https://slsa.dev)
 
 Flutter is Google's SDK for crafting beautiful, fast user experiences for
 mobile, web, and desktop from a single codebase. Flutter works with existing