[BUG] [Go] declared type cannot begin with numerals

Bug Report Checklist

• [ ] Have you provided a full/minimal spec to reproduce the issue?
  yes
• [ ] Have you validated the input using an OpenAPI validator ([example]
  (https://apidevtools.org/swagger-parser/online/))?
  yes
• [ ] What's the version of OpenAPI Generator used?
  4.0.0
• [ ] Have you search for related issues/PRs?
  yes
• [ ] What's the actual output vs expected output?

Actual :
package model
// Model5GInterfaceType : 5G interface type
type 5GInterfaceType string
// List of 5GInterfaceType
const (
N2 Model5GInterfaceType = "n2"
N3 Model5GInterfaceType = "n3"
)

Expected :

package model
// Model5GInterfaceType : 5G interface type
type Model5GInterfaceType string
// List of 5GInterfaceType
const (
N2 Model5GInterfaceType = "n2"
N3 Model5GInterfaceType = "n3"
)

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Description

openapi-generator version

OpenAPI declaration file content or url

Command line used for generation

java -jar ./openapi-generator-cli.jar generate -g go-server -i ./cmd/lizard-rest/yaml/test.yaml -o ./cmd/lizard-rest/generated/ --additional-properties packageName=model

test.txt

Steps to reproduce

Related issues/PRs

Suggest a fix

Start by reproducing the issue with the go-server generator command and the linked OpenAPI schema containing the 5GInterfaceType name. Check the generated Go model and verify that the declared type uses a valid identifier such as Model5GInterfaceType, with generation producing compilable output.