Upgrade Go to 1.24.3 (#31220)

* Upgrade Go to 1.24.3

Updates the following files:
- server/.go-version: 1.23.9 → 1.24.3
- server/build/Dockerfile.buildenv: golang:1.23.9-bullseye → golang:1.24.3-bullseye
- server/go.mod: go 1.23.0 → go 1.24.3, toolchain go1.23.9 → go1.24.3
- server/public/go.mod: go 1.23.0 → go 1.24.3, toolchain go1.23.9 → go1.24.3

Also fixes non-constant format string errors introduced by Go 1.24.3's stricter format string checking:
- Added response() helper function in slashcommands/util.go for simple string responses
- Removed unused responsef() function from slashcommands/util.go
- Replaced responsef() with response() for translated strings that don't need formatting
- Fixed fmt.Errorf and fmt.Fprintf calls to use proper format verbs instead of string concatenation
- Updated marketplace buildURL to handle format strings conditionally

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Update generated mocks for Go 1.24.3

Regenerated mocks using mockery v2.53.4 to ensure compatibility with Go 1.24.3.
This addresses mock generation failures that occurred with the Go upgrade.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Update to bookworm and fix non-existent sha

Signed-off-by: Stavros Foteinopoulos <stafot@gmail.com>

* fix non-constant format string

---------

Signed-off-by: Stavros Foteinopoulos <stafot@gmail.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Stavros Foteinopoulos <stafot@gmail.com>

server/go.mod

@@ -1,8 +1,6 @@
 module github.com/mattermost/mattermost/server/v8
 
-go 1.23.0
-
-toolchain go1.23.9
+go 1.24.3
 
 require (
 	code.sajari.com/docconv/v2 v2.0.0-pre.4