This PR:

• Adds isDate to CodegenModel
• adds isDate getter and setter to IJsonSchemaValidationProperties
• Adds a test to ensure that isDate has the correct values on on implementing java classes
• is a feature break out for [REQ] Implement needed common isX properties in java Schema classes in openapi core #7651

Note: this is potentially a breaking change because if one is generating a model for a date schema, the current master branch sets isString in that CodegenModel to true. After this PR, isString will be False and isDate will be true, which is consistent with how date schemas are handled in these classes:

• CodegenProperty
• CodegenParameter
• CodegenResponse

Setting isString and isDate for date schemas should be consistent in these 4 classes.

Checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• If contributing template-only or documentation-only changes which will change sample output, build the project beforehand.
• Run the shell script ./bin/generate-samples.shto update all Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master
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