Java: fix compatibilty with Java 1.8.

It's officially EOL, but there's extended support until 2030, and anyway
the Svix library advertises support for it. It's only this line that broke
compat, so it's an easy fix.

Author: tasn

java/lib/src/main/java/com/svix/Svix.java

@@ -6,6 +6,7 @@
 
 import okhttp3.HttpUrl;
 
+import java.util.HashMap;
 import java.util.Map;
 
 @Getter
@@ -60,10 +61,9 @@ public Svix(String token, SvixOptions options) {
             throw new IllegalArgumentException("Invalid base url");
         }
 
-        Map<String, String> defaultHeaders =
-                Map.of(
-                        "User-Agent", "svix-libs/" + Version.VERSION + "/java",
-                        "Authorization", "Bearer " + token);
+        Map<String, String> defaultHeaders = new HashMap<>();
+        defaultHeaders.put("User-Agent", "svix-libs/" + Version.VERSION + "/java");
+        defaultHeaders.put("Authorization", "Bearer " + token);
 
         SvixHttpClient httpClient =
                 new SvixHttpClient(parsedUrl, defaultHeaders, options.getRetrySchedule());