chore(mise): migrate gitleaks to betterleaks

gitleaks is in maintenance mode (security patches only); betterleaks is
the official successor by the same team and a drop-in replacement.

Author: mhiro2

mise/config.toml

@@ -2,6 +2,7 @@
 awscli = "latest"
 azure-cli = "latest"
 bat = "latest"
+betterleaks = "latest"
 bun = "latest"
 claude-code = "latest"
 codex = "latest"
@@ -12,7 +13,6 @@ fzf = "latest"
 gcloud = "latest"
 ghq = "latest"
 github-cli = "latest"
-gitleaks = "latest"
 gitui = "latest"
 go = "latest"
 gojq = "latest"