Deprecate DnsResolver in favor of AddressResolverGroup(#1572)

[young0264]

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Summary

This PR deprecates Lettuce's internal DNS resolver and removes the conditional logic that checks for the presence of Netty's DNS resolver on the classpath.

Closes #1527

Background

Until Lettuce 6.1, 'netty-resolvers-dns' was treated as an optional dependency and used conditionally if present on the classpath. However [issue #1527] and previous discussions(e.g., #1517) suggest that 'netty-resolver-dns' should become a required dependency starting from version 6.2.

When reviewing the latest codebase (currently 6.6), I found that:

• The code still uses 'LettuceClassUtils.isPresent(...)' to detect and optionally apply Netty DNS support
• Lettuce's custom 'DnsResolver' (based on blocking 'InetAddress') is still present
  Based on this history, I've made the following changes:

Changes

• Removed 'true' from 'netty-resolver-dns' in 'pom.xml'.
• Removed conditional logic in 'AddressResolverGroupProvider'
• Marked 'DnsResolver' (based on 'InetAddress') as '@deprecated'
• Documented the rationale and changes in an internal Google Doc

Let me know if additional coverage or documentation is needed.

Thanks!!

[tishun]

Thanks for the contribution!