Fix the lightsail name validation to reflect actual minimum length

[ryndaniels]

As the lightsail API documents, 2 characters is actually the minimum, not 3.

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Closes #10042

Release note for CHANGELOG:

BUG FIXES:
* resource/aws_lightsail_instance: Fixes an issue where 2-character lightsail instance names didn't get validated properly

Output from acceptance testing:

$ make testacc TESTARGS="-run=TestAccAWSLightsailInstance_"        
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./... -v -parallel 20 -run=TestAccAWSLightsailInstance_ -timeout 120m
?       github.com/terraform-providers/terraform-provider-aws   [no test files]
=== RUN   TestAccAWSLightsailInstance_basic
=== PAUSE TestAccAWSLightsailInstance_basic
=== RUN   TestAccAWSLightsailInstance_Name
=== PAUSE TestAccAWSLightsailInstance_Name
=== RUN   TestAccAWSLightsailInstance_Tags
=== PAUSE TestAccAWSLightsailInstance_Tags
=== RUN   TestAccAWSLightsailInstance_disapear
=== PAUSE TestAccAWSLightsailInstance_disapear
=== CONT  TestAccAWSLightsailInstance_basic
=== CONT  TestAccAWSLightsailInstance_disapear
=== CONT  TestAccAWSLightsailInstance_Name
=== CONT  TestAccAWSLightsailInstance_Tags
--- PASS: TestAccAWSLightsailInstance_disapear (81.48s)
--- PASS: TestAccAWSLightsailInstance_basic (83.75s)
--- PASS: TestAccAWSLightsailInstance_Tags (117.23s)
--- PASS: TestAccAWSLightsailInstance_Name (151.58s)
PASS
ok      github.com/terraform-providers/terraform-provider-aws/aws       152.726s

[aeschright]

🌟

--- PASS: TestAccAWSLightsailInstance_disapear (60.66s)
--- PASS: TestAccAWSLightsailInstance_basic (61.49s)
--- PASS: TestAccAWSLightsailInstance_Tags (71.99s)
--- PASS: TestAccAWSLightsailInstance_Name (105.44s)

[ghost]

This has been released in version 2.28.0 of the Terraform AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template for triage. Thanks!

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!