doc: Remove REQUIRED for boolean values

Setting REQUIRED on boolean values is misleading, as it may cause CSI
implementers to think that the field must be present in the GRPC payload
and that they should fail if it's not there, which is incorrect.

As mentioned in the GRPC Proto3 docs: "note that if a scalar message
field is set to its default, the value will not be serialized on the
wire."

This means that boolean fields will not be sent over the wire when their
value is set to false.  Making it impossible to know whether the GRPC
message had the field set in the first place.

This patch updates the comments to reflect the default value and removes
the misleading REQUIRED notation.

csi.proto

@@ -580,8 +580,7 @@ message ControllerPublishVolumeRequest {
   // This is a REQUIRED field.
   VolumeCapability volume_capability = 3;
 
-  // Whether to publish the volume in readonly mode. This field is
-  // REQUIRED.
+  // Whether to publish the volume in readonly mode. Defaults to false.
   bool readonly = 4;
 
   // Secrets required by plugin to complete controller publish volume
@@ -648,7 +647,7 @@ message ValidateVolumeCapabilitiesRequest {
 
 message ValidateVolumeCapabilitiesResponse {
   // True if the Plugin supports the specified capabilities for the
-  // given volume. This field is REQUIRED.
+  // given volume. Defaults to false.
   bool supported = 1;
 
   // Message to the CO if `supported` above is false. This field is
@@ -992,8 +991,7 @@ message NodePublishVolumeRequest {
   // This is a REQUIRED field.
   VolumeCapability volume_capability = 5;
 
-  // Whether to publish the volume in readonly mode. This field is
-  // REQUIRED.
+  // Whether to publish the volume in readonly mode. Defaults to false.
   bool readonly = 6;
 
   // Secrets required by plugin to complete node publish volume request.