Update Go version to 1.18

[hackerwins]

Description:

Update Go version to 1.18.

We need to check about changes from 1.17 to 1.18 and reflect the changes on the Yorkie.

https://tip.golang.org/doc/go1.18

Tasks

• [x] https://github.com/yorkie-team/yorkie/issues/234
• [x] Introduce Generics
  • [x] cache.go
  • [x] splay.go
  • [x] priority_queue
  • [x] llrb.go
  • [x] rga_tree_split

Why:

• We can reflect on the latest Go improvements and keep an up-to-date code.

[dc7303]

We can use generics while updating to 1.18.

I think the list below would be good if generics were applied.

• cache.go
• splay.go
• llrb.go
• rga_tree_split

[hackerwins]

@dc7303 Thanks for your comments. I added the tasks in the first comment.