Use packer-sdc in packer + remove mapstructure-to-hcl2 & struct-markdown

[azr]

This follows hashicorp/packer-plugin-sdk#53. This will be a breaking change for plugins that use go install .../hashicorp/packer/cmd/...@latest not when we merge this but when we release Packer because latest is the latest tagged version.

[codecov]

Codecov Report

Merging #10913 (89beb20) into master (da312e2) will increase coverage by 0.80%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
builder/alicloud/ecs/access_config.go	20.42% <ø> (+1.61%)	⬆️
builder/alicloud/ecs/builder.go	13.63% <ø> (+0.22%)	⬆️
builder/alicloud/ecs/image_config.go	59.25% <ø> (+2.11%)	⬆️
builder/alicloud/ecs/run_config.go	86.95% <ø> (+0.28%)	⬆️
builder/azure/arm/config.go	82.80% <ø> (+0.21%)	⬆️
builder/azure/chroot/builder.go	51.75% <ø> (-1.07%)	⬇️
...r/azure/chroot/shared_image_gallery_destination.go	90.62% <ø> (-1.05%)	⬇️
builder/azure/common/client/config.go	60.46% <ø> (+7.25%)	⬆️
builder/azure/dtl/config.go	55.64% <ø> (-0.39%)	⬇️
builder/cloudstack/config.go	82.72% <ø> (+1.14%)	⬆️
... and 946 more

[azr]

Mergin this now so that we get these changes before the plugin breakout. Makes our lives easier :D

[sylviamoss]

Sounds good!

[ghost]

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