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 #482 (481f04d) into main (5c0b0fa) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #482   +/-   ##
=======================================
  Coverage   86.37%   86.37%           
=======================================
  Files         145      145           
  Lines        6310     6310           
=======================================
  Hits         5450     5450           
  Misses        704      704           
  Partials      156      156           

Files	Coverage Δ
action/build/cancel.go	100.00% <ø> (ø)
action/build/get.go	94.44% <ø> (ø)
action/build/restart.go	100.00% <ø> (ø)
action/build/table.go	100.00% <ø> (ø)
action/build/validate.go	100.00% <ø> (ø)
action/build/view.go	100.00% <ø> (ø)
action/completion/generate.go	100.00% <ø> (ø)
action/completion/validate.go	100.00% <ø> (ø)
action/config/empty.go	88.67% <ø> (ø)
action/config/generate.go	90.16% <ø> (ø)
... and 107 more

... and 28 files with indirect coverage changes

[KellyMerrick]

lgtm