Prepare C++ code for Debian Bookworm, update C++ standard to C++-20

[uweseimet]

Debian Bookworm was released recently, and it's very likely just a matter of weeks until there is an official Raspberry Pi OS based on Bookworm. This is a list of changes and code improvements that can be applied:

• Drop the support for gcc < 10. The Bullseye default compiler is gcc 10, with Bookworm it's gcc 12. clang-13 the latest clang release on Bullseye, clang-15 the latest on Bookworm.
• With gcc 10 a C++-20 subset can be used as C++ standard level C++-20 provides several benefits see https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p2131r0.html and https://gcc.gnu.org/projects/cxx-status.html. Unfortunately gcc 10 does not support "using enum" and lacks some other C++-20 features.
• Clean up the code and make it better readable/maintainable by using C++-20 features, as far as supported by gcc 10
• Use filesystem::file_size(): bullseye 32 bit C++ toolchain supports files > 2 GiB
• Replace containers by spans where possible
• De-duplicate with templates
• Fix C++-20 issues reported by SonarCloud, as far as fixing with gcc 10 is possible
• Maybe use -flto for better optimization (binaries are much smaller, but compile/link time increases)

There is an "issue_1179" branch that reflects these changes. The code in this branch passes the test suite on a Pi with Bullseye and on a Pi that was updated from Bullseye to Bookworm.

>lsb_release -a
No LSB modules are available.
Distributor ID: Raspbian
Description:    Raspbian GNU/Linux 12 (bookworm)
Release:        12
Codename:       bookworm
>gcc --version
gcc (Raspbian 12.2.0-14+rpi1) 12.2.0
Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Note: This SonarCloud rule (applied for C++-20 only)

Transparent comparator should be used with associative "std::string" containers

should be disabled. It's not helpful/relevant for PiSCSI but just leads to rather complicated declarations of unordered maps.

[akuker]

Buster has been moved to "legacy" support, so seems like a reasonable change.

[uweseimet]

The plan is to wait for the officlal Raspberry Pi OS based on bookworm before merging the changes. Testing can already be done on an upgraded bullseye distribution.

[uweseimet]

I assume bookworm will be released together with the Pi 5, which is supposed to be released next month.