fix: make placeholder class public (#1085)

Currently javadoc build is failing:

```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:3.3.1:jar (attach-javadocs) on project google-cloud-bigtable-stats: MavenReportException: Error while generating Javadoc: 
[ERROR] Exit code: 1 - javadoc: error - No public or protected classes found to document.
```

google-cloud-bigtable-stats/src/main/java/com/google/cloud/bigtable/stats/MavenPlaceholderShaded.java

@@ -15,7 +15,7 @@
  */
 package com.google.cloud.bigtable.stats;
 
-final class MavenPlaceholderShaded {
+public final class MavenPlaceholderShaded {
   /**
    * This class is here to force generation of source javadoc jars so that the maven release process
    * doesn't complain. The shading plugin generated a shaded jar of bigtable-stats, but it doesn't