deps: Bump terraform-schema to 7b256e6c65f9

[radeksimko]

This brings in the following changes related to the upcoming 1.6 release:

• Add project attribute to workspaces block terraform-schema#257
• Update import ID to be interpolatable terraform-schema#260
• Add nullable to variable blocks for 1.1+ terraform-schema#261
• Add endpoint and domain to cos backend terraform-schema#262
• Add proxy_url to kubernetes backend terraform-schema#263
• backends: Update s3 backend w/ v1.6 changes terraform-schema#265
• Regenerate Terraform core versions terraform-schema#266
• schema/versions: Add 1.6.0 ahead of expected release terraform-schema#267

[github-actions]

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