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 #516 (039739f) into main (0650d90) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #516   +/-   ##
=======================================
  Coverage   83.44%   83.44%           
=======================================
  Files          69       69           
  Lines        5381     5381           
=======================================
  Hits         4490     4490           
  Misses        746      746           
  Partials      145      145           

Files	Coverage Δ
executor/context.go	100.00% <ø> (ø)
executor/executor.go	100.00% <ø> (ø)
executor/linux/api.go	13.95% <ø> (ø)
executor/linux/build.go	77.05% <ø> (ø)
executor/linux/driver.go	100.00% <ø> (ø)
executor/linux/linux.go	100.00% <ø> (ø)
executor/linux/opts.go	100.00% <ø> (ø)
executor/linux/secret.go	71.54% <ø> (ø)
executor/linux/service.go	80.15% <ø> (ø)
executor/linux/stage.go	86.71% <ø> (ø)
... and 59 more

[KellyMerrick]

lgtm