CI: update checks and copywrite config

[tgross]

Update CI so that we're checking and testing against the current stable and the previous stable. Run golangci-lint with current config and address the issues there. Add copywrite config and ensure it runs on every PR. Tidy up the makefile.

This should incidentally clear all the remaining CodeQL noise as well.

Ref: https://hashicorp.atlassian.net/browse/NMD-1566

[tgross]

GitHub Actions appears to be having a normal one, but:

$ make lint
golangci-lint run
0 issues.

$ make test
go test -v -race -timeout=60s -parallel=10 ./...
=== RUN   TestServer_StartStop
--- PASS: TestServer_StartStop (0.00s)
=== RUN   TestServer_Lookup
2026/06/11 13:25:37 [INFO] mdns: Failed to listen to both unicast and multicast on IPv6
--- PASS: TestServer_Lookup (0.05s)
=== RUN   TestNewMDNSService_BadParams
--- PASS: TestNewMDNSService_BadParams (0.00s)
=== RUN   TestMDNSService_BadAddr
--- PASS: TestMDNSService_BadAddr (0.00s)
=== RUN   TestMDNSService_ServiceAddr
--- PASS: TestMDNSService_ServiceAddr (0.00s)
=== RUN   TestMDNSService_InstanceAddr_ANY
--- PASS: TestMDNSService_InstanceAddr_ANY (0.00s)
=== RUN   TestMDNSService_InstanceAddr_SRV
--- PASS: TestMDNSService_InstanceAddr_SRV (0.00s)
=== RUN   TestMDNSService_InstanceAddr_A
--- PASS: TestMDNSService_InstanceAddr_A (0.00s)
=== RUN   TestMDNSService_InstanceAddr_AAAA
--- PASS: TestMDNSService_InstanceAddr_AAAA (0.00s)
=== RUN   TestMDNSService_InstanceAddr_TXT
--- PASS: TestMDNSService_InstanceAddr_TXT (0.00s)
=== RUN   TestMDNSService_HostNameQuery
--- PASS: TestMDNSService_HostNameQuery (0.00s)
=== RUN   TestMDNSService_serviceEnum_PTR
--- PASS: TestMDNSService_serviceEnum_PTR (0.00s)
PASS
ok      github.com/hashicorp/mdns       (cached)

[gulducat]

LGTM