Change Gradle property to "rename" (#528)

This changes the property to match the name of the CLI flag.

docs/modules/pkl-gradle/partials/gradle-codegen-properties.adoc

@@ -36,11 +36,11 @@ Example: `generateSpringBootConfig = true` +
 Whether to generate config classes for use with Spring Boot.
 ====
 
-.packageMapping: MapProperty<String, String>
+.renames: MapProperty<String, String>
 [%collapsible]
 ====
 Default: `[:]` +
-Example: `packageMapping = ["foo.": "com.example.foo.", "bar.Config": "com.example.bar.Config"]` +
+Example: `renames = ["foo.": "com.example.foo.", "bar.Config": "com.example.bar.Config"]` +
 Allows to change default class and package names (derived from Pkl module names) in the generated code.
 
 When you need the generated class or package names to be different from the default names derived from the Pkl module names, you can define a rename mapping, where the key is the original Pkl module name prefix, and the value is its replacement.
@@ -49,9 +49,9 @@ When you do, the generated code's `package` declarations, class names, as well a
 The prefixes are replaced literally, which means that dots at the end are important.
 If you want to rename packages only, in most cases, you must ensure that you have an ending dot on both sides of a mapping (except for an empty mapping, if you use it), otherwise you may get unexpected results:
 
-....
+----
 // Assuming the following mapping configuration:
-packageMapping = [
+renames = [
   "com.foo.": "x",  // Dot on the left only
   "org.bar": "y.",  // Dot on the right only
   "net.baz": "z"    // No dots
@@ -62,13 +62,13 @@ packageMapping = [
 "org.bar.baz" -> "y..baz"     // Double dot, invalid name
 "net.baz.qux" -> "z.qux"      // Looks okay, but...
 "net.bazqux"  -> "zqux"       // ...may cut the name in the middle.
-....
+----
 
 When computing the appropriate target name, the longest matching prefix is used:
 
-....
+----
 // Assuming the following mapping configuration:
-packageMapping = [
+renames = [
   "com.foo.Main": "w.Main",
   "com.foo.": "x.",
   "com.": "y.",
@@ -80,7 +80,7 @@ com.foo.Main -> w.Main
 com.foo.bar  -> x.bar
 com.baz.qux  -> y.baz.qux
 org.foo.bar  -> z.org.foo.bar
-....
+----
 
 Keys in this mapping can be arbitrary strings, including an empty string.
 Values must be valid dot-separated fully qualifed class name prefixes, possibly terminated by a dot.