feat: move default manylinux build to manylinux_2_28

[EwoutH]

Summary

This PR moves the default manylinux build from manylinux2014 to manylinux_2_28. The main motivation behind this is that CentOS 7, on which manylinux2014 is based, became end-of-life end of June. It also helps projects by defaulting to more performant wheels that have improved support for modern microarchitectures.

Motivation

The motivation to move the default manylinux build from manylinux2014 to manylinux_2_28 is two fold: On the one hand there's a necessity and on the other hand an oppertunity.

• CentOS 7 reached end-of-life in June 2024. This is already impacting users, as can be seem by steeply increased reports in the following (tracking) issues:
  • Update default image away from manylinux2014 #1772
  • Manylinux 2014 images no longer work due to CentOS 7 going EOL manylinux#1641
• manylinux_2_28 offers better performance and support for modern architectures, including AVX-512 support, new optimized mathematical functions, new floating point types, AArch64 and PowerPC optimizations, and much more.
• Support for manylinux_2_28 has steadily risen to 92.8% according to the manylinux timeline, and is expected to increase further over time.
  
  • According to glibc versions, the distro that don't support glibc 2.28 is mainly Amazon Linux 2, which I think they can handle (and a newer Amazon Linux 2023 image has been available for a while).

Implementation

• Updated default manylinux image references in cibuildwheel/options.py and cibuildwheel/resources/defaults.toml
• Modified documentation in docs/options.md to reflect the new default
• Adjusted test expectations in test/utils.py and unit_test/option_prepare_test.py

This PR closely follows the default move to musllinux_1_2 in #1817.

Impact

• Projects using cibuildwheel will now build manylinux_2_28 wheels by default
• Existing configurations specifying manylinux2014 or earlier will continue to work as before
• Users can still override the default to use other manylinux versions if needed

Closes #1772.

[EwoutH]

The remaining errors are about a manylinux_2_28 image for "i686" not existing. There are three ways to resolve this:

• Keep the default "i686" build on manylinux2014
• Stop building "i686" by default, and thus requiring "i686" to opt-in with a specific manylinux version.

I think for this PR the first options is most logical, while maybe on the long term the second options should be discussed.