Support Go 1.17, remove Go 1.14

[joeig]

• Does your submission pass all tests against mocks? make test
• Does your submission pass all tests against a real instance running PowerDNS Authoritative Server 4.1, 4.2 and 4.3? docker-compose -f docker-compose-v4.3.yml up && make test-without-mocks
• Do your tests meet or exceed the coverage results of the master branch? make coverage && go tool cover -html=c.out
• Does your submission pass the format guidelines? make check-fmt

Furthermore, the pipeline performs additional checks against your submission:

• Does your submission comply with common best practices? golangci-lint run && staticcheck ./...