ENH: Build manylinux2014 wheels by default

[tbirdso]

Updates default manylinux cross-compilation image from "_2_28" to "2014" which corresponds to toolset downgrade from glibc 2.28+ (Ubuntu 20.04+) to glibc 2.17+ (Ubuntu 18.04+).

This downgrade is made to better address existing community needs. Both "_2_28" and "2014" builds continue to be available and supported.

EDIT: Pending investigation of aarch64 failure in https://github.com/tbirdso/ITKSplitComponents/actions/runs/4284448724/jobs/7461422239

[tbirdso]

@thewtex I think the failure is due to only supporting _2_28 for aarch64 builds, hence switching the default from _2_28 to 2014 fails on aarch64. Given the roadmap we discussed today, probably the easiest path forward is to let this sit for now and include it with or after changes to support cross-compilation with manylinux2014-aarch64.