Remove obsolete Tuple<string, T>[] overloads from AddAllAsync

[imperugo]

Depends on #662.

Summary

Remove the three AddAllAsync<T>(Tuple<string, T>[] items, ...) overloads from IRedisDatabase and RedisDatabase, along with the internal Tuple-based ToRedisEntries<T> helper.

These overloads are deprecated in v13.5 (#662) in favour of the (string Key, T Value)[] ones, which avoid one heap allocation per item. This issue is the second half of that deprecation: the actual removal, which is why it lands in a major.

Preconditions

• Add ValueTuple overloads to AddAllAsync and deprecate the Tuple ones #662 shipped in v13.5, so consumers have had a full release cycle with a compile-time warning pointing at the replacement

Scope

• IRedisDatabase.cs — remove the 3 [Obsolete] overload declarations
• RedisDatabase.cs — remove the 3 [Obsolete] implementations
• ValueLengthExtensions.cs — remove the Tuple-based ToRedisEntries<T> helper kept for the deprecated path
• Tests — drop the cases covering the obsolete overloads, along with the #pragma warning disable CS0618 suppressions they needed
• doc/migration-v13-to-v14.md — document the removal and the one-line call-site fix (Tuple.Create(k, v) becomes (k, v))

Acceptance criteria

• No Tuple<string, T> remains anywhere in the public surface
• No CS0618 suppressions remain in the test project for this API
• Migration guide shows a before/after snippet
• Tests green on all TFMs