Explore using system-provided nlohmann-json

[SwooshyCueb]

At present, we use an externals package for nlohmann-json, specifically version 3.10.4.
Theoretically, we only need 3.6.1, though this has not been tested in quite some time:

irods/CMakeLists.txt

Line 214 in e4b4e8a

find_package(nlohmann_json "3.6.1" REQUIRED)

It may be possible for us to drop the externals package and use distro-provided packages instead; all distros we support (or plan to) have at least 3.6.1:

Distribution	Repository	Package name	Version	Package revision
EL7	EPEL	json-devel	3.6.1	2.el7
EL8	EPEL	json-devel	3.6.1	2.el8
EL9	EPEL	json-devel	3.11.3	1.el9
Ubuntu 20.04	universe	nlohmann-json3-dev	3.7.3	1
Ubuntu 22.04	universe	nlohmann-json3-dev	3.10.5	2
Ubuntu 24.04	universe	nlohmann-json3-dev	3.11.3	1
Debian 11	main	nlohmann-json3-dev	3.9.1	1
Debian 12	main	nlohmann-json3-dev	3.11.2	2

Let's give it a whirl.