chore: Add tag-prefix for java-server-sdk release collection

The java-core repo uses prefixed release tags
(e.g. launchdarkly-java-server-sdk-7.8.0) but the metadata
didn't specify a tag-prefix, so sdk-meta couldn't collect
releases after 7.7.0.

Author: aaron-zeisler

.sdk_metadata.json

@@ -9,6 +9,9 @@
         "Java"
       ],
       "userAgents": ["JavaClient"],
+      "releases": {
+        "tag-prefix": "launchdarkly-java-server-sdk-"
+      },
       "features": {
         "allFlags": { "introduced": "1.0" },
         "appMetadata": { "introduced": "5.8" },