Fix http → https for JSON Schema Store links in custom-templates.md

[Copilot]

All 5 references to the JSON Schema Store URL in docs/core/tools/custom-templates.md used http://json.schemastore.org/template. Updated them all to https://json.schemastore.org/template.

Affected locations:

• $schema field description table
• Prose link ("JSON Schema Store")
• Two code snippet examples ("$schema" values)
• "See also" reference link

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docs/core/tools/custom-templates.md	Custom templates for dotnet new

[Copilot]

Pull request overview

This PR updates the template.json JSON Schema Store references in the custom templates documentation to use secure https URLs.

Changes:

• Updated the $schema guidance and JSON examples to use https://json.schemastore.org/template.
• Updated the prose and “See also” links, but they currently point to the SchemaStore home page instead of the template schema URL.