Updating the debian/control file will automatically update the dependencies
in all CI environments that directly read the debian/control file, such
as Salsa-CI and buildbot.mariadb.org to some degree.
(https://github.com/MariaDB/mariadb.org-tools/issues/43)

On Debian/Ubuntu releases that don't have liburing-dev available,
automatically downgrade to libaio-dev (just like libcurl4->3 is done).
This ensures the debian/control file is always up-to-date and works for
latest Debian and Ubuntu releases, while the backwards compatibility mods
are maintained in autobake-deb.sh separately, and can be dropped from there
once support for certain platforms end.

Debian/Ubuntu availability visible at:
- https://packages.debian.org/search?searchon=names&keywords=liburing-dev
- https://packages.ubuntu.com/search?searchon=names&keywords=liburing-dev

Also modify debian/rules to force a build without libaio. Use YES instead
of ON to make the flag more logical (=turning libaio check "off").

Stop running Salsa-CI for Debian Stretch-backports, as it does not have
liburing-dev available nor is the old-old Debian stable a relevant platform
for MariaDB 10.6 to test against anymore. Since the Stretch-backports build
can no longer be made, neither can the MySQL 5.7 on Bionic upgrade test be
run, as it depended on the Stretch binary.

This commit does not modify the .travis.yml file, as Travis-CI does not
have new enough Ubuntu releases available yet. Also Travis-CI.org is
practically dead now as build times have been shrunk to near zero.

The scope of this change is also Debian/Ubuntu only. No RPM or Windows or
Mac changes are included in this commit.

This commit does not update the external libmariadb or ColumnStore
CI pipelines, as those are maintained in different repositories.