feat: upgrade to serverless v3 (#540)

BREAKING CHANGE: serverless v2 is no longer supported. It might still work, but bug fixes/new features will mostly be developed for and tested with serverless v3 (important security/bug fixes _may_ be back ported to v2 versions depending on the uptake of v3)

serverless v3 is now a peer dependency, you will get warnings if you are on earlier versions

example-service/package.json

@@ -11,6 +11,6 @@
   "license": "ISC",
   "devDependencies": {
     "@agiledigital/serverless-sns-sqs-lambda": "file:../",
-    "serverless": "^2.19.0"
+    "serverless": "^3.16.0"
   }
 }

example-service/serverless.yml

@@ -1,7 +1,4 @@
 service: sns-sqs-service
-# Opt-in to the strict config validation that will be default in v3
-# See: https://www.serverless.com/framework/docs/deprecations/#new-variables-resolver
-variablesResolutionMode: 20210219
 # Ensure validation issues are treated as errors
 configValidationMode: error