Identify / Remove EL7 & SLES12 accommodations

[dtrudg]

Type of issue

technical debt

Description of issue

RHEL7 is mainstream EOL in July 2024, followed by SLES12 in October 2024. These distributions have very old kernels and basic userspace libraries that require workarounds and restrictions on SingularityCE functionality. We can clean up code and reduce the number of paths that are taken by dropping support for these distributions.

Review the code to identify the special cases / branches for EL7 & SLES12.

Open sub-issues here to track their removal.

• Remove CentOS 7 from CI matrix: #2887
• Remove EL7, SLES 11 examples #2888
• Remove Yum CentOS7 bootstrap tests #2889
• Remove CentOS 7 regression test for issue 5250 #2890
• Remove CentOS 7 install specifics: #2891
• Remove kernel version as proxy requirement for OCSP test #2892
• Remove EL7 test case / handling of old ld listing #2893