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 #55 (693369d) into main (3171ea6) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main      #55   +/-   ##
=======================================
  Coverage   17.94%   17.94%           
=======================================
  Files           2        2           
  Lines         574      574           
=======================================
  Hits          103      103           
  Misses        463      463           
  Partials        8        8           

Files Changed	Coverage Δ
cmd/vela-slack/main.go	0.00% <0.00%> (ø)
cmd/vela-slack/plugin.go	81.10% <ø> (ø)