Add Slack access review scopes

Slack has two OAuth2 use cases in the app: the compliance page
integration (chat:write, channels:join, incoming-webhook) and the
access review driver that lists workspace members via users.list
(users:read, users:read.email). The per-caller scope refactor
already handled the compliance page but missed the access review
path, leaving it silently broken (zero scopes → missing_scope from
Slack API).

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/accessreview/drivers/oauth2_scopes.go

@@ -27,6 +27,7 @@ var providerOAuth2Scopes = map[coredata.ConnectorProvider][]string{
 	coredata.ConnectorProviderBrex:     {"openid", "offline_access"},
 	coredata.ConnectorProviderDocuSign: {"signature"},
 	coredata.ConnectorProviderLinear:   {"read"},
+	coredata.ConnectorProviderSlack:    {"users:read", "users:read.email"},
 	coredata.ConnectorProviderGoogleWorkspace: {
 		"https://www.googleapis.com/auth/admin.directory.user.readonly",
 		"https://www.googleapis.com/auth/admin.directory.group.member.readonly",