BIGTOP-4232. Drop Ubuntu-18.04

[masatana]

Description of PR

Ubuntu 18.04 has been already EOL. We can just drop it.

How was this patch tested?

$ ./gradlew bigtop-puppet -POS=ubuntu-18.04 -Pprefix=3.3.0 bigtop-puppet
(..snip...)
Dockerfile:5                                                                                                            --------------------                                                                                                       3 |     COPY puppetize.sh /tmp/puppetize.sh                                                                             4 |                                                                                                                     5 | >>> RUN bash /tmp/puppetize.sh
   6 |
--------------------
ERROR: failed to solve: process "/bin/sh -c bash /tmp/puppetize.sh" did not complete successfully: exit code: 1         
> Task :bigtop-puppet FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':bigtop-puppet'.
> Process 'command './build.sh'' finished with non-zero exit value 1

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org                                                                              
BUILD FAILED in 13s
1 actionable task: 1 executed

Failed as expected.

For code changes:

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