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 #145 (275ca0e) into main (b9a7bbd) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main     #145   +/-   ##
=======================================
  Coverage   47.52%   47.52%           
=======================================
  Files           6        6           
  Lines         425      425           
=======================================
  Hits          202      202           
  Misses        210      210           
  Partials       13       13           

Files Changed	Coverage Δ
cmd/vela-git/build.go	100.00% <ø> (ø)
cmd/vela-git/command.go	100.00% <ø> (ø)
cmd/vela-git/main.go	0.00% <0.00%> (ø)
cmd/vela-git/netrc.go	100.00% <ø> (ø)
cmd/vela-git/plugin.go	44.28% <ø> (ø)
cmd/vela-git/repo.go	100.00% <ø> (ø)

[chrispdriscoll]

LGTM