Remove centos6 from the build pipelines

Goal:

CentOS 6 went EOL on the 30th of November 2020, which is documented here.

We will stop shipping the binaries complied for CentOS6 starting from the next PMM release.

Example of code that needs to be fixed: https://github.com/Percona-Lab/jenkins-pipelines/blob/5c421d1cbf5292be739ad3aa3ea256493e7789f0/pmm/pmm2-client-autobuild.groovy#L113

Acceptance criteria:

• PMM Client & PMM Server are shipping binaries for CentOS versions starting from v7. 
• CentOS v6 is not used in the build pipelines.

Out of scope:

N/A 

Suggested implementation:

Search and remove all places in the build pipelines where we still build binaries for CentOS v6.

How to test:

During the next release verify that the binaries for CentOS v6 are not shipped.

Details: