bugfix: aws_lightsail_domain_entry id field update separator to , to support _ in txt records

[brittandeyoung]

Description

The current separator used for the custom id field uses _ as a separator. This causes issues when creating txt records which commonly have _ inside the record. This also updates the separator to be the same as the rest of the lightsail resources.

Relations

Relates #27843
Closes: #27927

References

Output from Acceptance Testing

$  make testacc TESTARGS='-run=TestAccLightsailDomainEntry' PKG=lightsail
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/lightsail/... -v -count 1 -parallel 20  -run=TestAccLightsailDomainEntry -timeout 180m
=== RUN   TestAccLightsailDomainEntry_basic
=== PAUSE TestAccLightsailDomainEntry_basic
=== RUN   TestAccLightsailDomainEntry_disappears
=== PAUSE TestAccLightsailDomainEntry_disappears
=== CONT  TestAccLightsailDomainEntry_basic
=== CONT  TestAccLightsailDomainEntry_disappears
--- PASS: TestAccLightsailDomainEntry_disappears (50.59s)
--- PASS: TestAccLightsailDomainEntry_basic (53.68s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/lightsail  55.779s

...

[github-actions]

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