FindBoost: Fix context discovery for 1.60 and below

* `all.hpp` was removed in 1.68:
  https://github.com/boostorg/context/commit/2e37599461912cc1679f106a25456a493f7f0b27
* `fcontext.hpp` was moved to detail in 1.61:
  https://github.com/boostorg/context/commit/c2f0dfdf2608c1ebaa2229fa6f358fe6aa103561

Fixes: #18126
8 jobs for fix_old_boost_context
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