Set version number for compile-testing to 0.21.0.

[release] Release Compile-Testing 0.20

Attempt to fix maven-javadoc-plugin, and remove redundant org.apache.…

…maven.plugins.

This turned out not to work, but it's probably a good idea, anyway.

To work around the actual problem, I am going to try switching from
JDK11 back to JDK8.

[release] Release Compile-Testing 0.18

[release] Release Compile Testing 0.17

[release] Release Truth 0.16

Set version number for compile-testing to 0.15.

Set version number for compile-testing to 0.14.

update version number

update version number