exoscale: migrate to API v2 endpoints

[kobajagi]

This PR reworks exoscale DNS provider to use exoscale API v2 endpoints.

No changes from #1674

Live tests passing:

$ go test -v -count 1 ./providers/dns/exoscale 
=== RUN   TestNewDNSProvider
=== RUN   TestNewDNSProvider/success
=== RUN   TestNewDNSProvider/missing_credentials
=== RUN   TestNewDNSProvider/missing_access_key
=== RUN   TestNewDNSProvider/missing_secret_key
--- PASS: TestNewDNSProvider (0.00s)
    --- PASS: TestNewDNSProvider/success (0.00s)
    --- PASS: TestNewDNSProvider/missing_credentials (0.00s)
    --- PASS: TestNewDNSProvider/missing_access_key (0.00s)
    --- PASS: TestNewDNSProvider/missing_secret_key (0.00s)
=== RUN   TestNewDNSProviderConfig
=== RUN   TestNewDNSProviderConfig/success
=== RUN   TestNewDNSProviderConfig/missing_credentials
=== RUN   TestNewDNSProviderConfig/missing_api_key
=== RUN   TestNewDNSProviderConfig/missing_secret_key
--- PASS: TestNewDNSProviderConfig (0.00s)
    --- PASS: TestNewDNSProviderConfig/success (0.00s)
    --- PASS: TestNewDNSProviderConfig/missing_credentials (0.00s)
    --- PASS: TestNewDNSProviderConfig/missing_api_key (0.00s)
    --- PASS: TestNewDNSProviderConfig/missing_secret_key (0.00s)
=== RUN   TestDNSProvider_FindZoneAndRecordName
=== RUN   TestDNSProvider_FindZoneAndRecordName/Extract_root_record_name
=== PAUSE TestDNSProvider_FindZoneAndRecordName/Extract_root_record_name
=== RUN   TestDNSProvider_FindZoneAndRecordName/Extract_sub_record_name
=== PAUSE TestDNSProvider_FindZoneAndRecordName/Extract_sub_record_name
=== CONT  TestDNSProvider_FindZoneAndRecordName/Extract_root_record_name
=== CONT  TestDNSProvider_FindZoneAndRecordName/Extract_sub_record_name
--- PASS: TestDNSProvider_FindZoneAndRecordName (0.00s)
    --- PASS: TestDNSProvider_FindZoneAndRecordName/Extract_root_record_name (0.01s)
    --- PASS: TestDNSProvider_FindZoneAndRecordName/Extract_sub_record_name (0.03s)
=== RUN   TestLivePresent
--- PASS: TestLivePresent (8.92s)
=== RUN   TestLiveCleanUp
--- PASS: TestLiveCleanUp (5.01s)
PASS
ok      github.com/go-acme/lego/v4/providers/dns/exoscale       13.964s

[kobajagi]

@ldez as advised in #1674, new PR is from my personal fork and not org.

[ldez]

Thank you 👍