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 #970 (c72fcfe) into main (f021c40) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #970   +/-   ##
=======================================
  Coverage   71.06%   71.06%           
=======================================
  Files         312      312           
  Lines       12996    12996           
=======================================
  Hits         9235     9235           
  Misses       3295     3295           
  Partials      466      466           

Files	Coverage Δ
api/build/cancel.go	0.00% <ø> (ø)
api/build/clean.go	0.00% <ø> (ø)
api/build/create.go	0.00% <ø> (ø)
api/build/delete.go	0.00% <ø> (ø)
api/build/executable.go	0.00% <ø> (ø)
api/build/get.go	0.00% <ø> (ø)
api/build/get_id.go	0.00% <ø> (ø)
api/build/list_org.go	0.00% <ø> (ø)
api/build/list_repo.go	0.00% <ø> (ø)
api/build/plan.go	0.00% <ø> (ø)
... and 78 more

... and 222 files with indirect coverage changes

[KellyMerrick]

lgtm