Update dependency github:grafana/flint to v0.21.0 (#2791)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: Jason Plumb <jplumb@splunk.com>

Author: renovate[bot]

mise.toml

@@ -1,8 +1,9 @@
 [tools]
+markdownlint-cli2 = "0.22.1"
+
 # Linters
-"github:grafana/flint" = "0.20.3"
+"github:grafana/flint" = "0.21.0"
 lychee = "0.23.0"
-markdownlint-cli2 = "0.22.1"
 
 [env]
 FLINT_CONFIG_DIR = ".github/config"