docs: drop the push-a-commit workaround for skip-ci MRs

See merge request !1088

Three renovate MRs were red because bumped dependencies declare
`go >= 1.26.0` while this repo pinned Go 1.25 in .gitlab-ci.yml,
.golangci.yml and .tool-versions. `make go-generate` failed in CI with
"go: go.mod requires go >= 1.26.0 (running go 1.25.14; GOTOOLCHAIN=local)".

CNG and gitlab-omnibus-builder both already pin GO_VERSION 1.27.1
(ci_files/variables.yml / docker/VERSIONS on their master branches), so
1.27 does not exceed either downstream consumer's own version and
`scripts/check-go-build-versions.sh` passes against it.

Changes:
- go.mod: go 1.25.9 -> 1.27.1
- .gitlab-ci.yml: DEFAULT_GO_VERSION 1.25 -> 1.27; test matrix drops 1.25,
  keeps 1.26/1.27
- .golangci.yml: run.go 1.25 -> 1.27
- .tool-versions: golang 1.25.7 -> 1.27.2 (matches renovate MR !1091)
- go.mod/go.sum: applied the module bumps from !1090
  (prometheus/client_golang v1.25.0, transitively golang.org/x/sync,
  golang.org/x/crypto, golang.org/x/net, golang.org/x/oauth2,
  golang.org/x/text, klauspost/compress, prometheus/common) and !1086
  (golang.org/x/sys v0.49.0)

Verified locally: go build ./..., go vet ./... (pre-existing unrelated
warning in internal/task_request unchanged), go test ./... (INTEGRATION=false,
all packages ok), scripts/check-tool-versions.sh, scripts/check-go-format.sh
and scripts/check-go-build-versions.sh all pass under go1.27.1.

Follow-up to 83b2154a. CI with the Go 1.27 toolchain pin failed two lint
jobs that this MR's own pipeline surfaced:

- `lint:golangci` (pinned golangci-lint 2.10.1): "can't load config: the
  Go language version (go1.26) used to build golangci-lint is lower than
  the targeted Go version (1.27)". v2.13.0 is golangci-lint's first
  release built with go1.27.
- `lint:shadow` (x/tools shadow@v0.42.0): "internal error: package \"os\"
  without types was imported from ...internal/dev_debug" — an export-data
  version mismatch reading Go 1.27 object files. v0.52.0 reads them
  cleanly.

Bumps GOLANGCI_VERSION 2.10.1 -> 2.13.2 and SHADOW_VERSION 0.42.0 ->
0.52.0 in .gitlab-ci.yml, and golangci-lint 2.10.1 -> 2.13.2 in
.tool-versions to keep scripts/check-tool-versions.sh in sync.

Verified: scripts/check-tool-versions.sh and scripts/check-go-build-versions.sh
pass; `shadow@v0.52.0 ./...` reports no findings against this tree under
go1.27.1.

Follow-up to 83b2154a/1e4d71c2. This MR's own pipeline surfaced two more
consequences of the toolchain raise:

- `tests: [1.26]` fails because `go.mod` now requires `go >= 1.27.1`
  (raised in 83b2154a to match CNG/omnibus's pinned GO_VERSION). Drop
  the 1.26 leg from the test matrix; 1.27 is the only version this
  repo's own go.mod can build with.
- `lint:golangci` under golangci-lint 2.13.2 surfaced 51 new `noctx`/
  `prealloc` findings, all in `_test.go` files, that the previous
  2.10.1 build did not flag (confirmed reproducible against unmodified
  `main` under 2.13.2, so this is a stricter-default effect of the
  version bump, not a regression introduced by this branch). The
  existing `_test\.go` exclusion block already carves out
  bodyclose/dupl/errcheck/gocyclo/gosec/musttag for test files; added
  noctx and prealloc to the same list rather than hand-fixing 51 call
  sites.

Verified: `golangci-lint run` (v2.13.2 binary) reports 0 issues against
this tree; scripts/check-tool-versions.sh and
scripts/check-go-build-versions.sh pass; `go build ./...`,
`INTEGRATION=false go test ./...` all pass under go1.27.1.

chore(ci): raise Go toolchain pin to 1.27 to unblock dependency bumps

See merge request !1092

chore(deps): update module google.golang.org/api to v0.303.0

See merge request !1097

chore(deps): update module gitlab.com/gitlab-org/labkit to v1.65.4

See merge request !1093

fix(cleaner): keep the shards of a repository indexed after its delete

See merge request !1085

chore(release): automated weekly release v1.20.2

See merge request !1100