RFR: 8316957: Serial: Change GenCollectedHeap to SerialHeap inside gc/serial folder

[Ivan Walulya]

On Tue, 26 Sep 2023 13:32:29 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Mostly renaming about a type. I kept the original var name `gch` to keep the diff smaller.

Marked as reviewed by iwalulya (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/15922#pullrequestreview-1648088471