Update ubuntu to v26 and move to clang-19

[EmilyBourne]

Update ubuntu to v26 and move to clang-19 to work around internal compiler error seen in #727.

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[github-actions]

This PR is failing tests so it has been put back into draft. Please remove the draft status when the tests pass.

[github-actions]

This PR is failing tests so it has been put back into draft. Please remove the draft status when the tests pass.

[EmilyBourne]

• Clang-19 KO
• Clang-21 KO
• Clang-22 KO
• Clang-22 + old packages (https://github.com/gyselax/gyselalibxx/actions/runs/30526511737/job/90818583777)
• Clang-22 + old packages + clang compilation of packages (https://github.com/gyselax/gyselalibxx/actions/runs/30530359544)
• Clang-22 + clang compilation of packages (https://github.com/gyselax/gyselalibxx/actions/runs/30530431056)