Raise minimum GCC version to 10.3

[cxzhong]

Summary

• raise the minimum supported system GCC, G++, and GFortran version to 10.3
• use full GNU compiler version output so the 10.3 boundary is evaluated correctly
• update the compiler documentation to recommend Ubuntu 22.04 (Jammy) and use GCC 15 in the installation example

Motivation

Current NumPy development requires GCC 10.3 or newer. Detecting older system compilers during configuration lets Sage select its bundled toolchain instead of failing later while building NumPy.

User impact

System GNU compilers older than 10.3 now trigger installation of Sage's bundled compiler toolchain. GCC 10.3 remains accepted. The Ubuntu instructions now describe a current Jammy-based GCC 15 setup and retain the ubuntu-toolchain-r/test PPA for releases whose standard repositories do not provide those packages.

Checks

• git diff --check
• boundary parsing: 10 and 10.2.1 rejected; 10.3.0 and 15.2.0 accepted
• local g++-15 and gfortran-15 full-version output verified as 15.2.0

[github-actions]

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