Drop support for GCC < 6.3 in Sage 9.7

[mkoeppe]

(see Meta-ticket #32074: Remove support for outdated distros)

This ticket can be used as a dependency for upgrade tickets that impose stricter compiler requirements.

CC: @dimpase @kliem

Component: build: configure

Author: Matthias Koeppe

Branch/Commit: 7c0072e

Reviewer: Jonathan Kliem

Issue created by migration from https://trac.sagemath.org/ticket/33316

[mkoeppe]

Dependencies: #33187

[mkoeppe]

Branch: u/mkoeppe/drop_support_for_gcc___6_3_in_sage_9_7

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

bf96a8a

README.md, src/doc/en/installation/source.rst: Update info on GCC versions

[sagetrac-git]

Commit: bf96a8a

[mkoeppe]

Author: Matthias Koeppe

[mkoeppe]

Work Issues: Update platforms tested on GH Actions, best on top of #33062

[sagetrac-git]

Changed commit from bf96a8a to 4e8b2c2

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

1124ded	build/pkgs/gcc/spkg-configure.m4: Bump minimum version to 6.3
4e8b2c2	README.md, src/doc/en/installation/source.rst: Update info on GCC versions

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

9b388ea

.github/workflows/tox*.yml: Update platforms for drop of GCC < 6.3

[sagetrac-git]

Changed commit from 4e8b2c2 to 9b388ea

[mkoeppe]

Changed work issues from Update platforms tested on GH Actions, best on top of #33062 to none

[mkoeppe]

Changed dependencies from #33187 to none

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

e7ff2a8

build/pkgs/gcc/spkg-configure.m4: Fixup

[sagetrac-git]

Changed commit from 9b388ea to e7ff2a8

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

0713f53

.github/workflows/tox*.yml: Replace ubuntu-xenial by ubuntu-xenial-toolchain-gcc_9