Make the default fallback weight in Tor 10.0

This is a follow-up to #24679 (moved).

  • update the default weight in parse_dir_fallback_line() to 10.0
  • update the man page to reflect the new default

10.0 gives us:

  • 0.5% clients bootstrapping off an authority when all fallbacks are up, * 2% when 25% are down (our replacement threshold)
  • 7% when 40% are down (our worst case scenario)
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