GHA: repair the linux-old job

package libc6_2.28-10+deb10u2_amd64.deb changed to
libc6_2.28-10+deb10u3_amd64.deb

Closes #13564

ci: add CMake build variation, fixup libssh detection in `linux-old`

To test without c-ares and hit `easy_lock.h` on an old system. Use this
new build step to introduce small variations,

ci: add CMake build variation, fixup libssh detection in `linux-old`

To test without c-ares and hit `easy_lock.h` on an old system. Use this
new build step to introduce small variations, and also test libssh2.

Also add workaround to existing job to enable libssh. (CMake's generic
auto-detection doesn't seem to work here.):
```
CMake Warning at CMakeLists.txt:908 (find_package):
Could not find a package configuration file provided by "libssh" with any
of the following names:

libsshConfig.cmake
libssh-config.cmake
```
Ref: https://github.com/curl/curl/actions/runs/8661316091/job/23750974358#step:5:69

Closes #13361

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cmake: fix `CURL_WERROR=ON` for old CMake and use it in GHA/linux-old

- cmake: fix `-pedantic-errors` for old CMake with `CURL_WERROR=ON` set.

`-pedantic-errors` option throws a w

cmake: fix `CURL_WERROR=ON` for old CMake and use it in GHA/linux-old

- cmake: fix `-pedantic-errors` for old CMake with `CURL_WERROR=ON` set.

`-pedantic-errors` option throws a warning with GCC (all versions) and
makes `check_symbol_exists()` fail in CMake versions older than
v3.23.0 (2022-03-29), when CMake introduced a workaround:

https://gitlab.kitware.com/cmake/cmake/-/issues/13208
https://gitlab.kitware.com/cmake/cmake/-/commit/eeb45401163d831b8c841ef6eba81466b4067b68
https://gitlab.kitware.com/cmake/cmake/-/commit/1ab7c3cd28b27ca162c4559e1026e5cad1898ade

Follow-up to 3829759bd042c03225ae862062560f568ba1a231 #12489

- set `CURL_WERROR=ON` for the `linux-old` job in CI.

Closes #13282

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GHA: enable unity mode for cmake jobs + tidy-ups

Unity mode is not supported by CMake v3.7.2 used in linux-old, but
enable it anyway for consistency and to kick in automatically once

GHA: enable unity mode for cmake jobs + tidy-ups

Unity mode is not supported by CMake v3.7.2 used in linux-old, but
enable it anyway for consistency and to kick in automatically once
migrating to a newer old Linux in the future.

Also:
- replace `CMAKE_COMPILE_WARNING_AS_ERROR` with `CURL_WERROR`.
- delete default build option `PICKY_COMPILER=ON`.

Closes #13277

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CI: Add CI build on Debian stretch to test old support

This version still has ELTS support and contains some old versions of
key components like cmake to help prevent us from breaking th

CI: Add CI build on Debian stretch to test old support

This version still has ELTS support and contains some old versions of
key components like cmake to help prevent us from breaking that support.

Closes #13029

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