fix(dns/jenkins.io) merge records to ensure SPF is valid

[dduportal]

Ref. jenkins-infra/helpdesk#4497

Before this PR, https://dmarcadvisor.com/fr/spf-check/ tells us:

[dduportal]

LGTM:

# azurerm_dns_txt_record.apex_jenkinsio will be updated in-place
  ~ resource "azurerm_dns_txt_record" "apex_jenkinsio" {
        id                  = "/subscriptions/dff2ec18-6a8e-405c-8e45-b7df7465acf0/resourceGroups/proddns_jenkinsio/providers/Microsoft.Network/dnsZones/jenkins.io/TXT/@"
        name                = "@"
        tags                = {
            "repository" = "jenkins-infra/azure-net"
            "scope"      = "terraform-managed"
        }
        # (4 unchanged attributes hidden)

      - record {
          - value = "v=spf1 include:mailgun.org ~all" -> null
        }
      - record {
          - value = "v=spf1 include:sendgrid.net -all" -> null
        }
      + record {
          + value = "v=spf1 include:mailgun.org include:sendgrid.net -all"
        }

        # (3 unchanged blocks hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.