Make action.yml compliant with JSON schema

These changes seem a bit silly, but they are indeed required by the official specification (even though even GitHub
doesn't bother to specify the `inputs.<input_id>.required` value for inputs with an `inputs.<input_id>.default` key in
their official workflows).

Author: per1234

action.yml

@@ -4,33 +4,43 @@ inputs:
   cli-version:
     description: 'Version of Arduino CLI to use when building'
     default: 'latest'
+    required: true
   fqbn:
     description: 'Full qualified board name, with Boards Manager URL if needed'
     default: 'arduino:avr:uno'
+    required: true
   libraries:
     description: 'YAML-format list of library dependencies to install'
     default: '- source-path: ./'
+    required: true
   platforms:
     description: 'YAML-format list of platform dependencies to install'
     default: ''
+    required: true
   sketch-paths:
     description: 'YAML-format list of paths containing sketches to compile.'
     default: '- examples'
+    required: true
   verbose:
     description: 'Set to true to show verbose output in the log'
-    default: false
+    default: 'false'
+    required: true
   sketches-report-path:
     description: 'Path in which to save a JSON formatted file containing data from the sketch compilations'
     default: 'sketches-reports'
+    required: true
   github-token:
     description: 'GitHub access token used to get information from the GitHub API. Only needed if you are using the deltas report feature in a private repository.'
     default: ''
+    required: true
   enable-deltas-report:
     description: 'Set to true to cause the action to determine the change in memory usage and compiler warnings of the compiled sketches between the head and base refs of a PR and the immediate parent commit of a push'
-    default: false
+    default: 'false'
+    required: true
   enable-warnings-report:
     description: 'Set to true to cause the action to record the compiler warning count for each sketch compilation in the sketches report'
-    default: false
+    default: 'false'
+    required: true
 
 runs:
   using: 'docker'