Migrate boost::filesystem to std::filesystem

[peterfpeterson]

This is a second round of many to move everything to using std::filesystem which is part of c++17. With this change, nothing links to the boost filesystem library anymore. As a bonus item, generating a fixed then string with random contents was moved into Kernel so it can be used more broadly.

Refs #37868. <!-- and fix #xxxx or close #xxxx xor resolves #xxxx. One line per issue fixed.

To test:

This is a refactor so looking at the code is enough.

This does not require release notes because it is a refactor which reduces what libraries are loaded/linked.

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