Fix issue 22213 - Base64: Missing @nogc attribute on encodeLength

[dkorpel]

Improve attributes of std.base64

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22213	normal	Base64: Missing `@nogc` attribute on encodeLength

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8430"