ci: use Depot runners for CI and release workflows

[ethanndickson]

Switches the CI and release workflows over to Depot runners, mirroring the pattern used in coder/coder. Each runs-on is guarded with github.repository_owner == 'coder' so forks continue to use ubuntu-latest.

Sizing

Picked per-job based on the actual workload:

Job	Runner	Why
build (test.yml)	depot-ubuntu-22.04-4	go build + golangci-lint, 5 min cap
generate (test.yml)	depot-ubuntu-22.04-4	go generate ./... (tfplugindocs), light
test (test.yml)	depot-ubuntu-22.04-4	10-way Terraform version matrix already parallelizes; per-shard work is container start + serial TF apply/destroy, not CPU-bound
lint (test.yml)	depot-ubuntu-22.04-4	make fmt + make gen diff check
goreleaser (release.yml)	depot-ubuntu-22.04-8	Cross-compiles ~15 OS/arch binaries + GPG-signs; Go cross-build parallelizes well

The 4-core size for the acceptance-test matrix is deliberate: matrix fan-out already gives horizontal parallelism, and bumping each shard to 8 cores would roughly 2× Depot minutes for that workflow without meaningfully shortening wall-clock.