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 #124 (a347834) into main (4429d0d) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main     #124   +/-   ##
=======================================
  Coverage   49.23%   49.23%           
=======================================
  Files           5        5           
  Lines         394      394           
=======================================
  Hits          194      194           
  Misses        198      198           
  Partials        2        2           

Files Changed	Coverage Δ
cmd/vela-downstream/build.go	100.00% <ø> (ø)
cmd/vela-downstream/config.go	100.00% <ø> (ø)
cmd/vela-downstream/main.go	0.00% <0.00%> (ø)
cmd/vela-downstream/plugin.go	51.92% <ø> (ø)
cmd/vela-downstream/repo.go	100.00% <ø> (ø)

[KellyMerrick]

lgtm