build(release): indicate repo is monorepo (#3181)

The `monorepoTags` flags tells `release-please` that multiple modules will be released from the same repository.
diff --git a/.github/release-please.yml b/.github/release-please.yml
index 9f6e4ac..a36ef87 100644
--- a/.github/release-please.yml
+++ b/.github/release-please.yml
@@ -1,3 +1,4 @@
 releaseType: go-yoshi
 handleGHRelease: true
 changelogPath: CHANGES.md
+monorepoTags: true