Replace to gopkg.in/yaml with github.com/goccy/go-yaml (slightly breaking change)

[bep]

Add this to have something to track this with.

The main blocker I see for this is that it's not possible to control location/timezone for dates without that information.

package main

import (
	"fmt"
	"log"

	v2 "gopkg.in/yaml.v2"
	v3 "gopkg.in/yaml.v3"
)

var data = `
d: 2021-07-15

`

func main() {
	mv2 := make(map[string]interface{})
	err := v2.Unmarshal([]byte(data), &mv2)
	if err != nil {
		log.Fatalf("error: %v", err)
	}
	fmt.Printf("--- mv2: %T %v \n", mv2["d"], mv2["d"])

	mv3 := make(map[string]interface{})
	err = v3.Unmarshal([]byte(data), &mv3)
	if err != nil {
		log.Fatalf("error: %v", err)
	}
	fmt.Printf("--- mv3: %T %v \n", mv3["d"], mv3["d"])
}

Prints:

--- mv2: string 2021-07-15 
--- mv3: time.Time 2021-07-15 00:00:00 +0000 UTC 

Note that v2 isn't perfect, either, but that at least allows us to handle known dates (e.g. the front matter dates).

[bep]

See go-yaml/yaml#770