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 #179 (27ca105) into main (083da87) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main     #179   +/-   ##
=======================================
  Coverage   44.91%   44.91%           
=======================================
  Files           7        7           
  Lines         688      688           
=======================================
  Hits          309      309           
  Misses        379      379           

Files Changed	Coverage Δ
cmd/vela-kaniko/build.go	100.00% <ø> (ø)
cmd/vela-kaniko/command.go	100.00% <ø> (ø)
cmd/vela-kaniko/image.go	100.00% <ø> (ø)
cmd/vela-kaniko/main.go	0.00% <0.00%> (ø)
cmd/vela-kaniko/plugin.go	96.21% <ø> (ø)
cmd/vela-kaniko/registry.go	100.00% <ø> (ø)
cmd/vela-kaniko/repo.go	100.00% <ø> (ø)

[chrispdriscoll]

LGTM