Merge bitcoin/bitcoin#28349: build: Require C++20 compiler

fa6e50d6c79633e22ad4cfc75f56aaa40112ecbb fuzz: Use C++20 starts_with in rpc.cpp (MarcoFalke)
faa48388bca06df1ca7ab92461b76a6720481e45 Revert "tracepoints: Disables `-Wgnu-zero-variadic-macro-arguments` to compile without warnings" (MarcoFalke)
fae3b77a87f4d799aca5907335a9dcbab3a51db6 refactor: Drop unused _Pragma to ignore -Wgnu-zero-variadic-macro-arguments (MarcoFalke)
fa02fc0a86c410f907de4fee91dd045547ea4b6e refactor: modernize-use-default-member-init for bit-fields (C++20) (MarcoFalke)
fa67f096bdea9db59dd20c470c9e32f3dac5be94 build: Require C++20 compiler (MarcoFalke)

Pull request description:

  C++20 allows to write safer code, because it allows to enforce more stuff at compile time (`constinit`, `conteval`, `constexpr`, `std::span`, ...).

  Also, it allows to write less verbose and easier to understand code (C++ 20 Concepts).

  See https://github.com/bitcoin/bitcoin/issues/23363 and https://en.cppreference.com/w/cpp/compiler_support#cpp20

  With g++-10 (https://github.com/bitcoin/bitcoin/pull/28348) and clang-13 (https://github.com/bitcoin/bitcoin/pull/28210), there is broad support for almost all features of C++20.

  It should be fine to require a C++20 compiler for Bitcoin Core 27.0 in 2024 (next year), not the soon upcoming 26.0 next month.

  This pull request includes three small cleanups to make use of C++20 features. If any issues are detected before or after merge, this should be easy to revert. If no issues arise, it should be fine to make use of more involved C++20 features later on.

ACKs for top commit:
  fanquake:
    ACK fa6e50d6c79633e22ad4cfc75f56aaa40112ecbb

Tree-SHA512: 244d79bfb0b750a4bdd713f40573b9ca33816fb84b6c84a58f027b9d7d4bb0cc4f18642959e4cf3d094808a69e5b8a327ca8521d7c0c08af27dacb5da3e78e71

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