build(deps): bump github.com/slack-go/slack from 0.20.0 to 0.21.0

Bumps [github.com/slack-go/slack](https://github.com/slack-go/slack) from 0.20.0 to 0.21.0.
- [Release notes](https://github.com/slack-go/slack/releases)
- [Changelog](https://github.com/slack-go/slack/blob/master/CHANGELOG.md)
- [Commits](slack-go/slack@v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: github.com/slack-go/slack
  dependency-version: 0.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

go.mod

@@ -4,7 +4,7 @@ go 1.25
 
 require (
 	github.com/kelseyhightower/envconfig v1.4.0
-	github.com/slack-go/slack v0.20.0
+	github.com/slack-go/slack v0.21.0
 )
 
 require github.com/gorilla/websocket v1.5.3 // indirect

go.sum

@@ -8,8 +8,8 @@ github.com/kelseyhightower/envconfig v1.4.0 h1:Im6hONhd3pLkfDFsbRgu68RDNkGF1r3dv
 github.com/kelseyhightower/envconfig v1.4.0/go.mod h1:cccZRl6mQpaq41TPp5QxidR+Sa3axMbJDNb//FQX6Gg=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
-github.com/slack-go/slack v0.20.0 h1:gbDdbee8+Z2o+DWx05Spq3GzbrLLleiRwHUKs+hZLSU=
-github.com/slack-go/slack v0.20.0/go.mod h1:K81UmCivcYd/5Jmz8vLBfuyoZ3B4rQC2GHVXHteXiAE=
+github.com/slack-go/slack v0.21.0 h1:TAGnZYFp79LAG/oqFzYhFJ9LwEwXJ93heCkPvwjxc7o=
+github.com/slack-go/slack v0.21.0/go.mod h1:K81UmCivcYd/5Jmz8vLBfuyoZ3B4rQC2GHVXHteXiAE=
 github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
 github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=