Update sbt-typelevel-mergify to 0.4.21 in series/0.9 (#870)

* Update sbt-typelevel-mergify to 0.4.21 in series/0.9

* Regenerate GitHub Actions workflow

Executed command: sbt githubWorkflowGenerate

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Co-authored-by: http4s-steward[bot] <106843772+http4s-steward[bot]@users.noreply.github.com>

.github/workflows/ci.yml

@@ -135,7 +135,6 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        scala: [2.13.10]
         java: [temurin@11]
     runs-on: ${{ matrix.os }}
     steps:
@@ -230,7 +229,7 @@ jobs:
           (echo "$PGP_PASSPHRASE"; echo; echo) | gpg --command-fd 0 --pinentry-mode loopback --change-passphrase $(gpg --list-secret-keys --with-colons 2> /dev/null | grep '^sec:' | cut --delimiter ':' --fields 5 | tail -n 1)
 
       - name: Publish
-        run: sbt '++ ${{ matrix.scala }}' tlCiRelease
+        run: sbt tlCiRelease
 
   site:
     name: Generate Site

project/plugins.sbt

@@ -1,2 +1,2 @@
 addSbtPlugin("org.http4s" % "sbt-http4s-org" % "0.14.12")
-addSbtPlugin("org.typelevel" % "sbt-typelevel-mergify" % "0.4.20")
+addSbtPlugin("org.typelevel" % "sbt-typelevel-mergify" % "0.4.21")