Update earthly/earthly Docker tag to v0.7.21 (#489)

[![Mend
Renovate](https://app.renovatebot.com/images/banner.svg)](https://renovatebot.com)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| earthly/earthly | patch | `v0.7.15` -> `v0.7.21` |

---

### Configuration

📅 **Schedule**: Branch creation - "after 11pm every weekday,before 7am
every weekday,every weekend" in timezone Europe/Brussels, Automerge - At
any time (no schedule defined).

🚦 **Automerge**: Enabled.

♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the
rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update
again.

---

- [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check
this box

---

This PR has been generated by [Mend
Renovate](https://www.mend.io/free-developer-tools/renovate/). View
repository job log
[here](https://developer.mend.io/github/kairos-io/provider-kairos).

<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiIzNy4zMS41IiwidXBkYXRlZEluVmVyIjoiMzcuMzEuNSIsInRhcmdldEJyYW5jaCI6Im1haW4ifQ==-->

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

earthly.sh

@@ -1,3 +1,3 @@
 #!/bin/bash
 
-docker run --privileged -v /var/run/docker.sock:/var/run/docker.sock --rm -t -v "$(pwd):/workspace" -v earthly-tmp:/tmp/earthly:rw earthly/earthly:v0.7.15 --allow-privileged "$@"
+docker run --privileged -v /var/run/docker.sock:/var/run/docker.sock --rm -t -v "$(pwd):/workspace" -v earthly-tmp:/tmp/earthly:rw earthly/earthly:v0.7.21 --allow-privileged "$@"