Fix glibc version requirement in documentation: manylinux_2_28 requires 2.28, not 2.27

[Copilot]

The documentation incorrectly stated that manylinux_2_28 wheels require glibc 2.27 or later and listed Ubuntu 18.04 as compatible. manylinux_2_28 actually requires glibc 2.28, and Ubuntu 18.04 ships with glibc 2.27.

Changes

• Corrected glibc requirement: 2.27 → 2.28 in system requirements
• Updated minimum Ubuntu version: 18.04 → 20.04 LTS (glibc 2.31)
• Added Ubuntu 18.04 to source-build list: Now documented alongside RHEL 7 and Ubuntu 16.04 as requiring source installation

Corrected Compatibility

Pre-built wheels work on:

• Ubuntu 20.04 LTS+ (glibc 2.31+)
• RHEL/CentOS 8+ (glibc 2.28+)
• Debian 10+ (glibc 2.28+)

Require building from source:

• Ubuntu 18.04 (glibc 2.27)
• Ubuntu 16.04 (glibc 2.23)
• RHEL/CentOS 7 (glibc 2.17)

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