[BUG] [kotlin-spring] Fixed escaped backticks in parameters using reserved words

[franciscobretal]

Changes:

• All mustache variables {{paramName}} have been changed to {{{paramName}}} so that their content is not escaped, thus fixing [BUG] [kotlin-spring] Escaped backticks in parameters using reserved words #18317.

Tests to check the problem is fixed:

1. Place a openapi.yml file containing the OpenAPI definition used as the bug example in the issue [BUG] [kotlin-spring] Escaped backticks in parameters using reserved words #18317.
2. Checkout this PR's branch.
3. Run the following CLI command. In this case, to ease testing, we use the -t flag to use the custom templates that have been edited in this branch.

openapi-generator-cli generate -i openapi.yml -g kotlin-spring -o out -t /path/to/openapi-generator/modules/openapi-generator/src/main/resources/kotlin-spring -p serviceImplementation=true -p interfaceOnly=true -p delegatePattern=true

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Closes #18317

[franciscobretal]

@jimschubert @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m @stefankoppier

This should be a quick and easy fix

[wing328]

lgtm

thanks for the fix