[C++][Docs] Update minimum GCC to 8 and C++ standard to C++20

[amoeba]

Describe the enhancement requested

This issue is to outline and track housekeeping items related to #44792 which updates the Arrow C++ codebase to require a C++20 compiler. This change implies we're also upgrading our minimum GCC version to 8 and we have a few housekeeping tasks related to all of it:

• Discuss and come to consensus on the dev mailing list about these changes (C++17 discussion for reference)
• Upate C++ codebase to C++20: [C++] compilation standard to C++20 #44792 (comment)
• Update the C++ build docs to document the new GCC and C++ standard policies as per this comment in [C++] compilation standard to C++20 #44792 (comment)
• Ensure dropping openSUSE 155 job doesn't lose us coverage as per this comment in [R][CI]: Compile error on opensuse155 #45718 (comment)
• Simplify code that exists purely for supporting openSUSE155 (1, 2)

Component(s)

C++, Documentation

[pitrou]

I've started the discussion on the dev ML:
https://lists.apache.org/thread/48zlj0dn2y0f53y2k37qsr90y781wfnj

[rkannan]

@amoeba @pitrou any update? The dev ML thread has multiple approvals. will the c++ standard be updated to c++20 in upcoming arrow release?

[kou]

Yes.