Add api diffs for 1.19.0 (#6921)

Following post-release instructions at
https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/RELEASING.md#update-release-versions-in-documentations

Will automate later once I understand this better.

docs/apidiffs/1.19.0_vs_1.18.0/opentelemetry-instrumentation-api.txt

@@ -0,0 +1,2 @@
+Comparing source compatibility of  against 
+No changes.