Remove consensus methods 25-27

According to proposal 290, we remove some consensus methods every time an LTS release is no longer supported: https://gitweb.torproject.org/torspec.git/tree/proposals/290-deprecate-consensus-methods.txt

When 0.2.9 is no longer supported, the earliest stable LTS will be 0.3.5.7, which supports consensus methods 25-28. Therefore, we can remove all consensus methods less than 28: https://gitweb.torproject.org/tor.git/tree/src/feature/dirauth/dirvote.h?h=tor-0.3.5.7#n60

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