test: Move cluster_linearize.h contents into cluster_linearize namespace #35737

Clang recently enabled -Wunused-template under -Wall (see https://github.com/llvm/llvm-project/pull/206123, https://github.com/llvm/llvm-project/pull/207848, https://github.com/llvm/llvm-project/pull/208001). Our codebase triggers some of these warnings.

This PR:

1. Avoids Clang's -Wunused-template warnings in src/test/util/cluster_linearize.h when building the bench_bitcoin and fuzz targets.

2. Follows the C++ Core Guidelines: "SF.21: Don't use an unnamed (anonymous) namespace in a header".

3. Drops [[maybe_unused]] annotations, which are no longer needed after changing the linkage from internal to external.

Along with #35679, this resolves all instances of this warning in the test/bench/fuzz code.

Another related change: https://github.com/bitcoin-core/minisketch/pull/102.

Steps to reproduce on the master branch @ 70d9ec7f3d452789d04dce81dc02db0b3b778bb5:

$ CXXFLAGS="-Wunused-template" cmake -B build -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DBUILD_BENCH=ON
$ cmake --build build -t test_bitcoin
$ cmake --build build -t bench_bitcoin
[19/62] Building CXX object src/bench/CMakeFiles/bench_bitcoin.dir/cluster_linearize.cpp.o
In file included from /home/hebasto/dev/bitcoin/src/bench/cluster_linearize.cpp:9:
/home/hebasto/dev/bitcoin/src/test/util/cluster_linearize.h:122:17: warning: unused function template 'Ser' [-Wunused-template]
  122 |     static void Ser(Stream& s, const DepGraph<SetType>& depgraph)
      |                 ^~~
/home/hebasto/dev/bitcoin/src/test/util/cluster_linearize.h:286:6: warning: unused function template 'SanityCheck' [-Wunused-template]
  286 | void SanityCheck(const DepGraph<SetType>& depgraph)
      |      ^~~~~~~~~~~
/home/hebasto/dev/bitcoin/src/test/util/cluster_linearize.h:383:6: warning: unused function template 'SanityCheck' [-Wunused-template]
  383 | void SanityCheck(const DepGraph<SetType>& depgraph, std::span<const DepGraphIndex> linearization)
      |      ^~~~~~~~~~~
3 warnings generated.
[62/62] Linking CXX executable bin/bench_bitcoin

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35737.

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