Update README with C++23 support and GCC 14 info #484

Job	Run time
check / clang-checks	1m 22s
check / editor-config	6s
build-os-matrix	4s
rpm-build (oraclelinux:10) / rpm-build	6m 17s
make-tests (oraclelinux:10) / tests	1m 45s
build (oraclelinux:10) / build	1m 56s
ipfix-elements-check / ipfix-elements-checks	24s
rpm-install (oraclelinux:10) / rpm-install	1m 2s
	12m 56s

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