Add performance warning to RedisTemplate#keys() and RedisOperations#keys() Javadoc

[connorcoco]

Description

This pull request updates the Javadoc of both RedisTemplate#keys() and RedisOperations#keys() methods to include a performance warning regarding the use of the KEYS command.

The KEYS command performs a full scan of the Redis keyspace, which can severely impact performance in production environments.
To avoid misuse, the updated Javadocs include a warning and recommend using scan(ScanOptions) as a safer alternative for large datasets.

• The warning has been added to:
  • RedisTemplate#keys() implementation method.
  • RedisOperations#keys() interface method.

Note: ReactiveRedisOperations#keys() already contains a similar warning and did not require changes.

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Checklist

• I have read the Spring Data contribution guidelines.
• I used the code formatters provided here and did not include any formatting-only changes.
• No test cases were needed as this change is limited to documentation (Javadoc only).
• No author header update was required since only a Javadoc comment was changed and no logic was modified.

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References

• Redis KEYS command
• scan(ScanOptions) alternative

[mp911de]

Care to move this documentation bit to RedisOperations? And please cross-check ReactiveRedisOperations for the same mention.

[connorcoco]

Care to move this documentation bit to RedisOperations? And please cross-check ReactiveRedisOperations for the same mention.

Thanks for the suggestion!
I reviewed ReactiveRedisOperations#keys() and confirmed it already includes a similar warning about performance concerns and recommends using scan().
Therefore, I only applied the Javadoc update to RedisOperations#keys().