Bump `base64ct` crate `1.5.0` -> `1.5.1`

This new release of base64ct checks for invalid symbols in non-padded inputs for decoding. Therefore, we can get rid of the logic implemented in !600 (merged), as pointed out by @Diziet here.

Note: there was a test that was incorrectly passing in the last implementation, so I've modified to fit expected behaviour. In particular, we were correctly decoding "Bg=" because the logic added the necessary extra = characters to be a valid padding... However, strictly speaking, the padding in this input is invalid, so the decoding should fail.