Change EL7 to EL8 as main testing platform

[tatb-vmware]

PXF 7 plans to deprecate CentOS 7 as a supported platform to be in line with Greenplum 7 compatibility.

This PR does the following:

• remove CentOS 7 as the main testing platform from the pipelines and replaces it with Rocky 8
• change upgrade test to upgrade from PXF 6 -> PXF 7

Other changes include:

• bootstrap the cluster migrate command to move the config files that were created in PXF_HOME in setup-pxf-6-latest task to PXF_BASE. This only applies to the upgrade task.
• remove PXF 5 because it is not longer used for the upgrade test for PXF 5 -> PXF 6
• remove conditional checks for PXF 5 since it will be removed
• use ${DEFAULT_OS_USER} instead of hardcoding OS name
• install nfs-utils in install_nfs.bash because the Rocky 8 image does not come with tools to set up NFS by default (CentOS 7 did come with said tools)
• standardize all the spacing in install_pxf.bash (before it was mixed spaces and tabs, now it is all tabs)