Avoid c-ares 1.34.7 and 1.34.8

These will silently freeze all DNS lookup as soon as the bug referred to in https://github.com/c-ares/c-ares/pull/1256 is hit.

I'm thinking of adding a build-time block for these versions in cmake, WDYT, @scylladb/seastar-maint ? How have we tackled issues like this in the past?