GraphQL: use latest tx hash for example query

[tom2drum]

Description and Related Issue(s)

Resolves #3008

This PR removes the NEXT_PUBLIC_GRAPHIQL_TRANSACTION environment variable and updates the GraphQL API documentation page to dynamically fetch the latest transaction hash from the API for the example query. If no transactions are available or if there's an error fetching the latest transaction, the component falls back to querying the zero address.

Proposed Changes

• Removed NEXT_PUBLIC_GRAPHIQL_TRANSACTION environment variable from all configuration files and validator schema
• Updated GraphQL.tsx component to fetch the latest transaction hash via the general:homepage_txs API endpoint
• Implemented fallback logic to query the zero address when the latest transaction cannot be retrieved

Environment Variable Changes

• Removed NEXT_PUBLIC_GRAPHIQL_TRANSACTION: This variable is no longer needed as the latest transaction hash is now obtained dynamically from the API. The GraphQL example query now uses the most recent transaction on the chain, or falls back to querying the zero address if no transactions are available.

Checklist for PR author

• I have tested these changes locally.
• I added tests to cover any new functionality, following this guide
• Whenever I fix a bug, I include a regression test to ensure that the bug does not reappear silently.
• If I have added, changed, renamed, or removed an environment variable
  • I updated the list of environment variables in the documentation
  • I made the necessary changes to the validator script according to the guide
  • I added "ENVs" label to this pull request