Upgrade gopls to v0.19.0, add make fix (#34772)

Upgrade to
[v0.19.0](https://github.com/golang/tools/releases/tag/gopls%2Fv0.19.0)
and fix issues. Runs with new `warning` serverity setting. This likely
does less checks than before. Additionally, add `make fix` which runs
modernize. This is also verified on CI.

For the record, here are the issues discoverd when running with `info`
severity, in case we want to fix these:

```
tests/integration/repo_test.go:95:5-14: could use tagged switch on i
tests/integration/api_packages_generic_test.go:149:4-64: could use tagged switch on setting.Packages.Storage.Type
services/webhook/msteams_test.go:33:4-33: could use tagged switch on fact.Name
services/webhook/msteams_test.go:59:4-33: could use tagged switch on fact.Name
services/webhook/msteams_test.go:85:4-33: could use tagged switch on fact.Name
services/webhook/msteams_test.go:111:4-33: could use tagged switch on fact.Name
services/webhook/msteams_test.go:138:4-33: could use tagged switch on fact.Name
services/webhook/msteams_test.go:161:4-33: could use tagged switch on fact.Name
services/webhook/msteams_test.go:187:4-33: could use tagged switch on fact.Name
services/webhook/msteams_test.go:213:4-33: could use tagged switch on fact.Name
services/webhook/msteams_test.go:239:4-33: could use tagged switch on fact.Name
services/webhook/msteams_test.go:266:4-33: could use tagged switch on fact.Name
services/webhook/msteams_test.go:407:4-33: could use tagged switch on fact.Name
tests/integration/api_packages_conan_test.go:350:6-33: could use tagged switch on pf.Name
models/issues/tracked_time_test.go:98:3-18: could use tagged switch on user.ID
tests/integration/api_token_test.go:505:5-43: could use tagged switch on minRequiredLevel
services/gitdiff/gitdiff.go:220:33-46: method "getLineLegacy" is unused
```

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

Makefile

@@ -36,7 +36,8 @@ XGO_PACKAGE ?= src.techknowlogick.com/xgo@latest
 GO_LICENSES_PACKAGE ?= github.com/google/go-licenses@v1
 GOVULNCHECK_PACKAGE ?= golang.org/x/vuln/cmd/govulncheck@v1
 ACTIONLINT_PACKAGE ?= github.com/rhysd/actionlint/cmd/actionlint@v1
-GOPLS_PACKAGE ?= golang.org/x/tools/gopls@v0.17.1
+GOPLS_PACKAGE ?= golang.org/x/tools/gopls@v0.19.0
+GOPLS_MODERNIZE_PACKAGE ?= golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@v0.19.0
 
 DOCKER_IMAGE ?= gitea/gitea
 DOCKER_TAG ?= latest
@@ -230,7 +231,7 @@ clean: ## delete backend and integration files
 		tests/e2e/reports/ tests/e2e/test-artifacts/ tests/e2e/test-snapshots/
 
 .PHONY: fmt
-fmt: ## format the Go code
+fmt: ## format the Go and template code
 	@GOFUMPT_PACKAGE=$(GOFUMPT_PACKAGE) $(GO) run build/code-batch-process.go gitea-fmt -w '{file-list}'
 	$(eval TEMPLATES := $(shell find templates -type f -name '*.tmpl'))
 	@# strip whitespace after '{{' or '(' and before '}}' or ')' unless there is only
@@ -249,6 +250,19 @@ fmt-check: fmt
 	  exit 1; \
 	fi
 
+.PHONY: fix
+fix: ## apply automated fixes to Go code
+	$(GO) run $(GOPLS_MODERNIZE_PACKAGE) -fix ./...
+
+.PHONY: fix-check
+fix-check: fix
+	@diff=$$(git diff --color=always $(GO_SOURCES)); \
+	if [ -n "$$diff" ]; then \
+	  echo "Please run 'make fix' and commit the result:"; \
+	  printf "%s" "$${diff}"; \
+	  exit 1; \
+	fi
+
 .PHONY: $(TAGS_EVIDENCE)
 $(TAGS_EVIDENCE):
 	@mkdir -p $(MAKE_EVIDENCE_DIR)
@@ -288,7 +302,7 @@ checks: checks-frontend checks-backend ## run various consistency checks
 checks-frontend: lockfile-check svg-check ## check frontend files
 
 .PHONY: checks-backend
-checks-backend: tidy-check swagger-check fmt-check swagger-validate security-check ## check backend files
+checks-backend: tidy-check swagger-check fmt-check fix-check swagger-validate security-check ## check backend files
 
 .PHONY: lint
 lint: lint-frontend lint-backend lint-spell ## lint everything
@@ -809,6 +823,7 @@ deps-tools: ## install tool dependencies
 	$(GO) install $(GOVULNCHECK_PACKAGE) & \
 	$(GO) install $(ACTIONLINT_PACKAGE) & \
 	$(GO) install $(GOPLS_PACKAGE) & \
+	$(GO) install $(GOPLS_MODERNIZE_PACKAGE) & \
 	wait
 
 node_modules: package-lock.json