Merge #18364: random: remove getentropy() fallback for macOS < 10.12

f9f210d8de43b4559fe7c80bb286aeb60de52b54 doc: fix GetTimeMicros() comment in random.cpp (fanquake)
a8897115626ab6509c67511e50e73c0f7c953c6a rand: remove getentropy() fallback for macOS < 10.12 (fanquake)

Pull request description:

  We [no longer support macOS < 10.12](https://github.com/bitcoin/bitcoin/pull/17550) (our binaries will not run), so remove the fallback for when `getentropy()` wasn't available. From the manpage:

  ```bash
  HISTORY
       The getentropy() function appeared in OSX 10.12
  ```

  Note that compiling on macOS you'll see a new unused function warning:
  ```bash
  random.cpp:256:13: warning: unused function 'GetDevURandom' [-Wunused-function]
  static void GetDevURandom(unsigned char *ent32)
              ^
  1 warning generated.
  ```

  This will likely be addressed as part of #17563.

ACKs for top commit:
  vasild:
    ACK f9f210d8 (code review, not tested)
  elichai:
    utACK f9f210d8de43b4559fe7c80bb286aeb60de52b54
  practicalswift:
    ACK f9f210d8de43b4559fe7c80bb286aeb60de52b54 -- patch looks correct
  laanwj:
    ACK f9f210d8de43b4559fe7c80bb286aeb60de52b54
  hebasto:
    ACK f9f210d8de43b4559fe7c80bb286aeb60de52b54, tested on macOS 10.13.6: compiled

Tree-SHA512: 6bd2a721f23605a8bca0b7b51f42d628ebf92a18e74eb43194331ba745ee449223aff84119892781c40b188c70b75417447f4e390e3d9ac549292de2b1e8b308

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