build: remove glibc backcompat

[tobtoht]

This PR bumps the minimum required glibc version for release binaries to 2.27 from 2.17.

Operating System	glibc version	EOL
Ubuntu 20.04	2.31	2025-04
Debian 10	2.28	2024-07
openSUSE 15.4	2.31	2023-12
CentOS 7	2.17	2024-06

After 10 years, CentOS 7 is finally nearing EOL.

This is needed for #8929.

[selsta]

What about Ubuntu 18.04, what glibc version does it use? I know it's EOL but I think it's somewhat still used.

[tobtoht]

~ docker run --rm -it ubuntu:18.04 bash
root@6be149847463:/# ldd --version
ldd (Ubuntu GLIBC 2.27-3ubuntu1.6) 2.27

[moneromooo-monero]

Why is it needed for #8929 ?

[tobtoht]

I went over this again and found that -DBACKCOMPAT=ON only lowers the minimum glibc to 2.17 for x86_64 builds with Gitian. At some point backcompat appears to have broken for other targets (riscv64 never supported < 2.27) and as far as I know nobody has complained.

Arch	v0.18.3.1	#8929 (w/ BACKCOMPAT)	#8929
x86_64	2.17	2.23	2.27
aarch64	2.23	2.23	2.27
i686	2.27	2.27	2.27
arm	2.27	2.27	2.27
riscv64	2.27	n/a	2.27

We could build #8929 with -DBACKCOMPAT=ON and this would lower the minimum glibc for x86_64 and aarch64 to 2.23 instead of 2.27, so this PR it is not strictly necessary.

However, with no supported Linux distributions using a glibc < 2.27 (or even < 2.31) by the time we branch from master it does not seem worthwhile to continue to fix and maintain the related depends patches and glibc_compat.cpp.

[moneromooo-monero]

Thanks (no objection to the patch)