Bump base64 from 0.2.0 to 0.3.0 in /steep

Bumps [base64](https://github.com/ruby/base64) from 0.2.0 to 0.3.0.
- [Release notes](https://github.com/ruby/base64/releases)
- [Commits](ruby/base64@v0.2.0...v0.3.0)

---
updated-dependencies:
- dependency-name: base64
  dependency-version: 0.3.0
  dependency-type: indirect
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

steep/Gemfile.lock

@@ -15,7 +15,7 @@ GEM
       tzinfo (~> 2.0, >= 2.0.5)
       uri (>= 0.13.1)
     ast (2.4.3)
-    base64 (0.2.0)
+    base64 (0.3.0)
     benchmark (0.4.0)
     bigdecimal (3.1.9)
     concurrent-ruby (1.3.5)