Backport Candidates – v0.7.3

[offa]

Candidates: https://github.com/offa/influxdb-cxx/issues?q=label%3Abackport+milestone%3Av0.8.0

• [x] Conan Center badge
• [x] Deprecate Conanfile / build in favour of Conan Center: #233
• [x] Fix for broken test tag: https://github.com/offa/influxdb-cxx/commit/b31f94982fd1d50e89ce04f66c694bec108bf470
• [x] Fix OS X CI build: #239 #240
• [x] #252
• [x] Fix Linux CI build:
  • [x] #254 / #255
  • [x] #257
• [ ] #264
• [x] #266
• [x] #221
• [x] #213