[BUG][crystal][client] gitignore.mustache lists ruby gem files instead of crystal shard files

Created by: cyangle

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

generated crystal project's gitignore lists ruby gem files instead of crystal shard files

openapi-generator version

All versions supporting crystal client, it's not a regression

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

generate crystal client project with pet_store api spec

Related issues/PRs

Suggest a fix

Update gitignore.mustache with crystal shard related files