Replace deprecated std::is_pod

[nlohmann]

This PR replaces std::is_pod by std::is_trivial.

Closes #1913

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[nlohmann]

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🔖 Release item

This issue/PR will be part of the next release of the library. This template helps preparing the release notes.

Type

• ✨ New Feature
• 🐛 Bug Fix
• ⚡️ Improvement
• 🔨 Further Change
• 🔥 Deprecated function

Description

• Replace deprecated std::is_pod calls. std::is_pod is deprecated in C++20 #1913 Replace deprecated std::is_pod #2033

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