docs: broken and outdated links in README, SECURITY.md, CONTRIBUTING.md, and env files

[Sertug17]

Description

Several links across the repository are broken (404) or outdated (redirect to new URLs):

Broken (404)

1. README.md line 46 https://basechaindata.vercel.app returns 404
2. SECURITY.md line 35 https://docs.optimism.io/docs/releases/bedrock/ returns 404

Outdated (redirect)

3. CONTRIBUTING.md line 55 https://support.coinbase.com/customer/en/portal/articles/2288496-how-can-i-contact-coinbase-support- redirects to https://help.coinbase.com/en
4. .env.mainnet line 53 & .env.sepolia line 53 https://reth.rs/run/faq/pruning/ redirects to https://reth.rs/run/storage/pruning

Suggested replacements

1. https://basechaindata.vercel.app → https://docs.base.org/base-chain/node-operators/snapshots
2. https://docs.optimism.io/docs/releases/bedrock/ → https://docs.optimism.io/stack/getting-started
3. Coinbase support link → https://help.coinbase.com/en
4. Reth pruning link → https://reth.rs/run/storage/pruning

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[kutluhaneth46]

Taking the remaining broken/outdated links from this issue (README snapshot URL, SECURITY.md Bedrock docs URL, CONTRIBUTING.md Coinbase support URL). #1173 already covers the reth pruning env-file links.