Fix docs for cache ttl env var

[mrfelton]

This pull request includes a series of changes to ensure consistency in the naming of the CACHE_STD_TTL configuration option across the codebase. The most important changes include updating the configuration option name in the README.md file and several instances within the src/server.tsx file.

Configuration updates:

• README.md: Renamed CACHE_STDTTL to CACHE_STD_TTL in the configuration options table.

Code updates:

• src/server.tsx: Updated the CACHE_STDTTL constant to CACHE_STD_TTL in the configuration retrieval statement.
• src/server.tsx: Updated the CACHE_STDTTL constant to CACHE_STD_TTL in the DataProviderManager service initialization.
• src/server.tsx: Updated the CACHE_STDTTL constant to CACHE_STD_TTL in the cache headers setting for the root endpoint.
• src/server.tsx: Updated the CACHE_STDTTL constant to CACHE_STD_TTL in the cache headers setting for the /v1/fee-estimates endpoint.