Use constexpr values in both is_bgl_named_params specializations.

[varunsha30]

Before submitting

• This PR targets the develop branch.
• I searched for an existing PR or issue covering the same change.
• My contribution is licensed under the Boost Software License 1.0.

Type of change

• Bug fix
• New feature or API addition
• Refactor (no behavior change)
• Documentation
• Build, CI, or tooling
• Other (specify below)

Does this PR introduce a breaking change?

• Yes (describe migration impact below)
• No

What this PR does

Replaces BOOST_STATIC_CONSTANT with static constexpr in both specializations of detail::graph::is_bgl_named_params. The trait continues to distinguish named-parameter calls from centrality-map overloads.

Motivation

part of #593

Testing

tested using cxxstd=14 and toolset=clang.
device: macbook M2 air.

Checklist

• Existing tests pass (b2 in the test/ directory).
• New behavior is covered by a test, or this is a docs / build / refactor change.
• Documentation was updated if user-facing behavior changed.
• No new compiler warnings on the platforms I built against.

[github-actions]

Boost dependency footprint vs develop (auto-generated).
PR run 37483820671 vs develop run 37447552382 (d5110c4e71).

Header-inclusion weights (graph files pulling each direct dependency in):

No header-inclusion-weight changes.

Transitive Boost modules: 47 → 47 (0)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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