Simplify Travis-CI file and extend inline comments.

Upgrade to using Ubuntu 20.04 (Focal) as the baseline distro version
now that Travis-CI has made it available. Drop Xenial and all the
excess repositories Xenial needed. Now we only Focal and one Bionic
build to keep things simple and streamlined.

Keep GCC-7/Clang-7 as the older compiler, and start using GCC-10
and Clang-10 as the newer compiler. Assume that if both of them
build OK, than the intermediate versions would be OK as well.

Print 'apt-cache policy' to make it transparent in build logs what
repositories was used for build dependencies.

Remove temporary workaround from homebrew install step as Travis-CI has
fixed the original issue.

Revert ignoring results form build that previously failed on the test
main.thread_pool_info as MDEV-20372 is not fixed.

Keep arm64 failures ignored due to MDEV-23955.

Allow failures for the test main.column_compression 'innodb' due
to MDEV-23954.