fix str.removeprefix only works after python 3.9

BoringSSL-Package.swift

@@ -7,6 +7,8 @@ let basePath = "third_party/boringssl-with-bazel"
 let privacyInfoPath = "../../src/objective-c/PrivacyInfo.xcprivacy"
 let testPath = "test/boringssl_spm_build"
 
+
+
 let package = Package(
   name: "BoringSSL-GRPC",
   products: [

templates/BoringSSL-Package.swift.template

@@ -9,6 +9,8 @@
     let privacyInfoPath = "../../src/objective-c/PrivacyInfo.xcprivacy"
     let testPath = "test/boringssl_spm_build"
     
+
+
     let package = Package(
       name: "BoringSSL-GRPC",
       products: [
@@ -25,12 +27,14 @@
           exclude: [
           ],
         <%
+          def remove_prefix(s, prefix):
+            return s[len(prefix):] if s.startswith(prefix) else s
           files = []
           lib_maps = {lib.name: lib for lib in libs}
           for dep in swift_boringssl_package.get('deps', []):
             lib = lib_maps[dep]
             files.extend(lib.src)
-          files = sorted(set({file.removeprefix("third_party/boringssl-with-bazel/"): file for file in files}))
+          files = sorted(set({remove_prefix(file, "third_party/boringssl-with-bazel/"): file for file in files}))
         %>
           sources: [
             % for file in files: