Remove deprecated Utf8StringCodec class (#3328)

[Kiminni]

This PR removes the deprecated Utf8StringCodec class as discussed in #3328.

Important Note

Existing code that uses Utf8StringCodec should migrate to StringCodec.UTF8 for identical functionality:

• StringCodec.UTF8 provides the same UTF-8 encoding behavior
• All existing functionality remains available through the standard StringCodec implementation
• Performance characteristics are identical

This approach removes the deprecated class while providing a clear migration path. All existing tests continue to pass without modification.

This PR is a follow-up to #1137 and addresses part of #3328.

Changes Made

• Removed src/main/java/io/lettuce/core/codec/Utf8StringCodec.java
• Removed src/test/java/io/lettuce/core/Utf8StringCodecIntegrationTests.java
• Removed src/test/jmh/io/lettuce/core/codec/Utf8StringCodecBenchmark.java
• Updated CommandBenchmark.java to use StringCodec.UTF8 instead of new Utf8StringCodec()
• Cleaned up unused imports in test files

[tishun]

LGTM

Thanks for the contribution!