Stop supporting implicit PRUNE data storage mode

[lucassaldanha]

Last release, we have added a warning for users that are relying on implicit PRUNE data storage mode configuration (see #8369).

Now, we can remove any support for implicit PRUNE mode. Our default behavior will be solely MINIMAL.

Scenarios:

• If the user has --data-storage-mode option, use the selected mode;
• If the user does not have --data-storage-mode option, use MINIMAL;

(In both cases, we are gonna keep writing the file data-storage-mode.txt, solely to support a safe downgrade path to previous versions of Teku. The file will not be read by newer Teku versions)

Tasks

• update logic to stop supporting implicit PRUNE mode
• update documentation
• prepare/execute comms plan

Reference:

• Startup error: No enum constant tech.pegasys.teku.storage.server.StateStorageMode #8357