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 #161 (929f1ee) into main (27beb3a) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main     #161   +/-   ##
=======================================
  Coverage   59.74%   59.74%           
=======================================
  Files           9        9           
  Lines         621      621           
=======================================
  Hits          371      371           
  Misses        249      249           
  Partials        1        1           

Files Changed	Coverage Δ
cmd/vela-kubernetes/apply.go	98.46% <ø> (ø)
cmd/vela-kubernetes/command.go	100.00% <ø> (ø)
cmd/vela-kubernetes/config.go	100.00% <ø> (ø)
cmd/vela-kubernetes/container.go	100.00% <ø> (ø)
cmd/vela-kubernetes/kubectl.go	78.37% <ø> (ø)
cmd/vela-kubernetes/main.go	0.00% <0.00%> (ø)
cmd/vela-kubernetes/patch.go	98.91% <ø> (ø)
cmd/vela-kubernetes/plugin.go	62.06% <ø> (ø)
cmd/vela-kubernetes/status.go	98.33% <ø> (ø)

[chrispdriscoll]

LGTM