Simplify snippet names

[gewarren]

Since these snippets are in a subfolder name "system-text-json", it makes sense to remove that part of the next subfolder name.

This PR doesn't change anything else (except for bullet points that were automatically changed from - to * or vice versa).

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📄 File	🔗 Preview link
docs/standard/datetime/system-text-json-support.md	DateTime and DateTimeOffset support in System.Text.Json
docs/standard/serialization/system-text-json/character-casing.md	How to enable case-insensitive property name matching with System.Text.Json
docs/standard/serialization/system-text-json/character-encoding.md	How to customize character encoding with System.Text.Json
docs/standard/serialization/system-text-json/configure-options.md	docs/standard/serialization/system-text-json/configure-options
docs/standard/serialization/system-text-json/converters-how-to.md	How to write custom converters for JSON serialization (marshalling) in .NET
docs/standard/serialization/system-text-json/customize-properties.md	How to customize property names and values with System.Text.Json
docs/standard/serialization/system-text-json/handle-overflow.md	How to handle overflow JSON or use JsonElement or JsonNode in System.Text.Json
docs/standard/serialization/system-text-json/how-to.md	docs/standard/serialization/system-text-json/how-to
docs/standard/serialization/system-text-json/ignore-properties.md	How to ignore properties with System.Text.Json
docs/standard/serialization/system-text-json/immutability.md	How to use immutable types and non-public accessors with System.Text.Json
docs/standard/serialization/system-text-json/invalid-json.md	How to allow some kinds of invalid JSON with System.Text.Json
docs/standard/serialization/system-text-json/migrate-from-newtonsoft.md	docs/standard/serialization/system-text-json/migrate-from-newtonsoft
docs/standard/serialization/system-text-json/polymorphism.md	How to serialize properties of derived classes with System.Text.Json
docs/standard/serialization/system-text-json/preserve-references.md	How to preserve references in System.Text.Json
docs/standard/serialization/system-text-json/source-generation.md	How to use source generation in System.Text.Json
docs/standard/serialization/system-text-json/supported-collection-types.md	docs/standard/serialization/system-text-json/supported-collection-types
docs/standard/serialization/system-text-json/use-dom.md	How to use a JSON document object model in System.Text.Json
docs/standard/serialization/system-text-json/use-utf8jsonreader.md	How to use Utf8JsonReader in System.Text.Json
docs/standard/serialization/system-text-json/use-utf8jsonwriter.md	How to use Utf8JsonWriter in System.Text.Json

[IEvangelist]

Looks good to me, I verified all the *.md files and saw that the rest were just moves.