chore(license): update source code headers + copyright year

[wass3rw3rk]

ref: go-vela/community#747

• replaces code file license headers with SPDX header (see https://spdx.dev/ids/#why); this removes copyright + year
• update golangci-lint rule to ensure code file enforcement
• use first commit date for copyright year and updates year in other select places
• format is changed to match "Copyright [yyyy] [name of copyright owner]" as shown here: https://www.apache.org/licenses/LICENSE-2.0.html

[codecov]

Codecov Report

Merging #100 (177a405) into main (2e11d17) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main     #100   +/-   ##
=======================================
  Coverage   63.71%   63.71%           
=======================================
  Files           7        7           
  Lines         667      667           
=======================================
  Hits          425      425           
  Misses        236      236           
  Partials        6        6           

Files Changed	Coverage Δ
cmd/vela-docker/build.go	97.26% <ø> (ø)
cmd/vela-docker/command.go	68.75% <ø> (ø)
cmd/vela-docker/daemon.go	75.96% <ø> (ø)
cmd/vela-docker/main.go	0.00% <0.00%> (ø)
cmd/vela-docker/plugin.go	17.74% <ø> (ø)
cmd/vela-docker/push.go	96.42% <ø> (ø)
cmd/vela-docker/registry.go	100.00% <ø> (ø)