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 #53 (402b39a) into main (59f9348) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #53   +/-   ##
=======================================
  Coverage   62.12%   62.12%           
=======================================
  Files           3        3           
  Lines         462      462           
=======================================
  Hits          287      287           
  Misses        145      145           
  Partials       30       30           

Files Changed	Coverage Δ
internal/npm/config.go	79.24% <ø> (ø)
internal/npm/package.go	93.75% <ø> (ø)
internal/npm/plugin.go	58.52% <ø> (ø)

[KellyMerrick]

lgtm