Use the new warning control instead of unsafeFlags (#1882)

## Motivation

In my recent work to build `protoc` from source I mistakenly disabled
warnings by using unsafe flags. Unsafe flags make packages essentially
unusable.

## Modifications

Disable the warnings by using the new warning control settings
introduced in 6.2 instead.

## Result

Fixes #1881

Author: FranzBusch

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.10
+// swift-tools-version:6.2
 
 // Package.swift
 //
@@ -350,10 +350,8 @@ let package = Package(
                 "protobuf/upb_generator/minitable/names_internal.cc",
             ],
             cSettings: [
-                .unsafeFlags([
-                    "-Wno-conversion",
-                    "-Wno-deprecated-declarations",
-                ])
+                .disableWarning("conversion"),
+                .disableWarning("deprecated-declarations"),
             ],
             cxxSettings: [
                 .headerSearchPath("abseil/"),
@@ -363,10 +361,8 @@ let package = Package(
                 .headerSearchPath("protobuf/upb/"),
                 .headerSearchPath("protobuf/upb_generator/"),
                 .define("UPB_BOOTSTRAP_STAGE", to: "0"),
-                .unsafeFlags([
-                    "-Wno-conversion",
-                    "-Wno-deprecated-declarations",
-                ]),
+                .disableWarning("conversion"),
+                .disableWarning("deprecated-declarations"),
             ],
             linkerSettings: [
                 .linkedFramework(