[Firestore] Migrate deprecated Abseil types and attributes to C++17 standard library

[cherylEnkidu]

Description

Following the migration to C++17 as the minimum language standard (#15074), this PR modernizes Firestore Core by migrating deprecated Abseil attributes and types to their standard C++17 equivalents:

1. [[maybe_unused]] Attribute:

   • Replaced deprecated ABSL_ATTRIBUTE_UNUSED with C++17 [[maybe_unused]] in iterator_adaptors_test.cc.

2. std::void_t Migration:

   • Replaced absl::void_t with std::void_t in Firestore core utility headers (iterator_adaptors.h, type_traits.h, to_string.h, and comparison.h).
   • Removed obsolete #include "absl/meta/type_traits.h" headers.

3. std::optional Migration:

   • Replaced absl::optional, absl::nullopt, and absl::make_optional with standard std::optional, std::nullopt, and std::make_optional across 29 Firestore core source files in api/, bundle/, core/, local/, model/, and remote/.
   • Included <optional> where needed and removed unneeded Abseil type includes.

Testing

• Verified Swift package manifest syntax (swift package dump-package).
• Verified header compilation and syntax with Clang C++17 (-std=c++17).

API Changes

None. These are internal implementation cleanups only.

#no-changelog

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