CI - Add required reviewers to testnet environments

[MantisClone]

Problem

The testnet GitHub environments (Sepolia, NEAR testnet, Mantle testnet) only have branch_policy protection, unlike other environments (mainnet, base, optimism, etc.) which have required reviewers. This allowed deployments to auto-execute without approval, creating unpublished deployments that broke The Graph Studio's /version/latest alias.

Proposed Solution

Add required reviewers to all testnet environments to match production environments:

1. Go to: https://github.com/RequestNetwork/payments-subgraph/settings/environments
2. Click on "sepolia" environment → Add "Required reviewers" protection rule
3. Click on "near-testnet" environment → Add "Required reviewers" protection rule
4. Click on "mantle-testnet" environment → Add "Required reviewers" protection rule

Considerations

• This adds an extra approval step for testnet deployments
• Consistent with how other environments are configured
• Prevents accidental deployments that could break downstream consumers (requestNetwork SDK)