Use thermite 0.14's version option instead of monkeypatching thermite

[danielpclark]

Stacked on #187 (the base branch here; it retargets to master once that merges). Goes with danielpclark/thermite#2, and must not be merged before thermite 0.14.0 is on rubygems.org, since the gemspec now requires thermite ~> 0.14.

What

lib/faster_path/thermite_initialize.rb redefined two thermite methods, and both were doing the wrong thing with the thermite the fork now has:

• Config#ruby_version put the full Ruby version into the release tarball names (ruby3.4.6), so a prebuilt extension could only be downloaded by that exact patch release, although it works on every 3.4.x. thermite's own ruby34 naming is what its download code expects.
• Tasks#github_download_uri made downloads use the gem's version as the release tag instead of the crate's. thermite master removed that method in its refactor, so the override had become dead code and installs without Cargo would have looked for v0.1.0. thermite 0.14 adds a version option for exactly this; the Rakefiles pass version: FasterPath::VERSION, so tarballs are faster_path-0.5.0-ruby34-linux-x86_64.tar.gz under tag v0.5.0.

rake build_lib also cleaned the Cargo target through Tasks#run_cargo, removed by the same refactor; it uses Thermite::Cargo now. The thermite test in the suite now checks what the Rakefiles rely on (tarball naming, the clean step) instead of the monkeypatch targets. The "method redefined" warnings on every rake run are gone with the file.

The Gemfile takes thermite from the fork's branch until 0.14.0 is published; that line can go once it is.

Verification

Ruby 3.4.6 on Linux: rake build_lib thermite:tarball builds and produces faster_path-0.5.0-ruby34-linux-x86_64.tar.gz; TEST_MONKEYPATCHES=true WITH_REGRESSION=true ENCODING=true rake minitest lint passes (138 tests, no offenses).

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