Update garbage-collector.md

[Maoni0]

added some clarification to the standalone GC configs. describes the behavior in .net8 and prior vs .net9. also added a header for each config since there are multiple in this section.

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docs/core/runtime-config/garbage-collector.md	Runtime configuration options for garbage collection

[Maoni0]

CC @cshung.

[cshung]

I confirm the content is accurate.

[Maoni0]

do you have any feedback, @gewarren ?