chore(librarian): update codecov targets for several languages (#4801)

The code coverage targets for the Go, Java, and Node.js librarian
implementations are updated in codecov.yml. These changes reflect recent
improvements in coverage for these languages.

The updated targets move the minimum required coverage closer to the 80%
goal specified in the outstanding issues for these implementations.

For #4662
For #4663
For #4664

Author: julieqiu

codecov.yml

@@ -45,23 +45,23 @@ coverage:
       librarian-golang:
         # TODO(https://github.com/googleapis/librarian/issues/4662): fix
         # coverage back to 80%
-        target: 69%
+        target: 78%
         threshold: 0%
         if_ci_failed: error
         paths:
           - internal/librarian/golang/**
       librarian-java:
         # TODO(https://github.com/googleapis/librarian/issues/4663): fix
         # coverage back to 80%
-        target: 73%
+        target: 74%
         threshold: 0%
         if_ci_failed: error
         paths:
           - internal/librarian/java/**
       librarian-nodejs:
         # TODO(https://github.com/googleapis/librarian/issues/4664): fix
         # coverage back to 80%
-        target: 49%
+        target: 58%
         threshold: 0%
         if_ci_failed: error
         paths: