chore: bump Go to 1.26.0

[yxxhero]

No description provided.

[yxxhero]

Run golangci/golangci-lint-action@v9
Restore cache
Install
run golangci-lint
Running [/home/runner/golangci-lint-2.7.2-linux-amd64/golangci-lint config path] in [/home/runner/work/helm-diff/helm-diff] ...
Running [/home/runner/golangci-lint-2.7.2-linux-amd64/golangci-lint run] in [/home/runner/work/helm-diff/helm-diff] ...
Error: can't load config: the Go language version (go1.25) used to build golangci-lint is lower than the targeted Go version (1.26.0)
The command is terminated due to an error: can't load config: the Go language version (go1.25) used to build golangci-lint is lower than the targeted Go version (1.26.0) @copilot

[Copilot]

Run golangci/golangci-lint-action@v9
Restore cache
Install
...

The golangci-lint version has been bumped from v2.7.2 to v2.12.2 in e6a2c93. The v2.12.2 release is built with Go 1.26 (as confirmed in golangci-lint's release workflow), so it correctly supports projects targeting go1.26.0.

[yxxhero]

@copilot fix ci issue.

[Copilot]

@copilot fix ci issue.

Fixed in 8c748ec. The CI was failing because golangci-lint v2.12.2 enforces stricter goconst checks — flagging string literals where named constants already exist. Changes made:

• cmd/upgrade.go: Added dryRunNone, dryRunServer, envFalse constants; replaced all dry-run string literals with constants
• cmd/helm.go: Replaced "client", "true", "false", "server" literals with the new constants
• cmd/helpers.go: Replaced "true" literal with the existing envTrue constant
• diff/diff.go: Added kindSecret = "Secret" constant; replaced all "Secret" literals with it
• .golangci.yaml: Added goconst to the test file exclusion rules