fix(contracts): OZ-L02 Lack of gap Variable

[zimpha]

Purpose or design rationale of this PR

This PR fixed the issue reported by Openzepplin (L-01 Misleading Comment) during Scroll USDC Gateway audit. The following are the details:

The CCTPGatewayBase contract does not contain a gap variable although it is upgradeable.

Consider adding a gap variable following OpenZeppelin's upgradeable contracts guide to avoid future storage collisions.

PR title

Your PR title must follow conventional commits (as we are doing squash merge for each PR), so it must start with one of the following types:

• fix: A bug fix

Deployment tag versioning

Has tag in common/version.go been updated or have you added bump-version label to this PR?

• No, this PR doesn't involve a new deployment, git tag, docker image tag
• Yes

Breaking change label

Does this PR have the breaking-change label?

• No, this PR is not a breaking change
• Yes

[github-actions]

LCOV of commit 47db95a during Contracts #31

Summary coverage rate:
  lines......: 51.6% (1013 of 1963 lines)
  functions..: 65.6% (227 of 346 functions)
  branches...: no data found

Files changed coverage rate: n/a