* add functions for returning the max and min Envoy major versions
- added an UnsupportedEnvoyVersions list
- removed an unused error from TestDetermineSupportedProxyFeaturesFromString
- modified minSupportedVersion to use the function for getting the Min Envoy major version. Using just the major version without the patch is equivalent to using `.0`
* added a function for executing the envoy --version command
- added a new exec.go file to not be locked to unix system
* added envoy version check when using consul connect envoy
* added changelog entry
* added docs change
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