docs: specify false as default for new inputs

The inputs expect `true` to enable the copying behavior. It wasn't
really clear from the action.yml nor from the readme that these are
boolean flags. By specifying the default as `false` this becomes clear.

Ultimately, an empty input would still disable the copying behavior, but
that's less relevant than the knowledge that this is a boolean flag.

This change doesn't affect the usage or implementation.

README.md

@@ -107,7 +107,7 @@ The action can be configured with the following optional [inputs](https://docs.g
 
 ### `copy_assignees`
 
-Default: `''` (disabled)
+Default: `false` (disabled)
 
 Controls wheather the assignees of the original pull request should be added to the backports.
 
@@ -121,13 +121,13 @@ By default, no labels are copied.
 
 ### `copy_milestone`
 
-Default: `''` (disabled)
+Default: `false` (disabled)
 
 Controls wheather the "milestone" of the original pull request should be added to the backports.
 
 ### `copy_reviewers`
 
-Default: `''` (disabled)
+Default: `false` (disabled)
 
 Controls wheather the reviewers of the original pull request should be added to the backports.
 

action.yml

@@ -7,6 +7,7 @@ inputs:
     description: >
       Whether or not to copy the assignees from the original pull request to the backport pull request.
       By default, the assignees are not copied.
+    default: false
   copy_labels_pattern:
     description: >
       Regex pattern to match github labels which will be copied from the original pull request to the backport pull request.
@@ -16,10 +17,12 @@ inputs:
     description: >
       Whether or not to copy the milestone from the original pull request to the backport pull request.
       By default, the milestone is not copied.
+    default: false
   copy_reviewers:
     description: >
       Whether or not to copy the reviewers from the original pull request to the backport pull request.
       By default, the reviewers are not copied.
+    default: false
   github_token:
     description: >
       Token to authenticate requests to GitHub.