Update Rust crate base64 to ^0.21.3

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
base64	dependencies	patch	^0.21.2 -> ^0.21.3

---

Release Notes

marshallpierce/rust-base64 (base64)

### [`v0.21.3`](https://togithub.com/marshallpierce/rust-base64/blob/HEAD/RELEASE-NOTES.md#0213) [Compare Source](https://togithub.com/marshallpierce/rust-base64/compare/v0.21.2...v0.21.3) - Implement `source` instead of `cause` on Error types - Roll back MSRV to 1.48.0 so Debian can continue to live in a time warp - Slightly faster chunked encoding for short inputs - Decrease binary size

---

Configuration

📅 Schedule: Branch creation - At any time (no schedule defined), 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.

---

• [ ] If you want to rebase/retry this PR, check this box

---

This PR has been generated by Mend Renovate. View repository job log here.