PHPC-1131 changed the AppVeyor build matrix to PHP 7.1 through 7.4.

Per AppVeyor's logs, our PHP 7.1 builds spend 4x the time running our test suite than other versions. I'm not sure why this is, as we don't have detailed timing information for each test, but I think we can consider removing PHP 7.1 from the build matrix altogether. Note that PECL itself only builds DLLs for 7.2 - 7.4 (likely tied to actively supported versions).