Missing `backwardCompatibilityCheck` configuration flag documentation

[boesing]

Summary

Flag was introduced with 1.23.0 but I actually missed to properly add it to the documentation (README.md) though it is part of the configuration schema:

laminas-ci-matrix-action/laminas-ci.schema.json

Lines 342 to 347 in 35dab19

	"backwardCompatibilityCheck": {
	"type": "boolean",
	"title": "Flag to enable/disable backwards compatibility check",
	"description": "This flag enables/disables backwards compatibility check using roave/backward-compatibility-check.",
	"default": false
	},