[Task] @Deprecated lock subsystem is still the wired production default

## Description
The entire `shenyu-admin.lock` subsystem is `@Deprecated` ("Please use `JdbcLockRegistry#obtain` / `DefaultLockRepository`") but remains the live `@Bean`.

## Location
```
shenyu-admin.lock.RegisterExecutionLock.java:27
RegisterTransactionUtil.java:29
ForUpdateBackedRegisterExecutionLock.java:33
PlatformTransactionRegisterExecutionRepository.java:31
RegisterExecutionRepository.java:26
RegisterCenterConfiguration.java:79-80 (new PlatformTransactionRegisterExecutionRepository(...) as @Bean)
```

## Impact
Marking it deprecated while shipping it as default misleads both users and tooling — risk of "removing dead code" that is actually live.

## Suggested fix
Either migrate the bean to `JdbcLockRegistry`/`DefaultLockRepository` and delete the subsystem, or drop the `@Deprecated` until migration is real.

## Related existing issue(s)
None

_Identified during the 2026-08-02 audit; full list in [`docs/issue-candidates-2026-08-02.md`](docs/issue-candidates-2026-08-02.md)._

Start at RegisterCenterConfiguration.java:79-80 and trace the @Bean through the listed shenyu-admin.lock classes. Compare that production wiring with JdbcLockRegistry#obtain and DefaultLockRepository before confirming with maintainers whether to migrate and remove the deprecated subsystem or remove its deprecation. Done means the chosen path is implemented consistently and the live default is no longer misleading.

Written by the indexing model from the issue text.