refactor: Change usage from Mnemonic to MnemonicHelper from Apollo

[hamada147]

Overview

We don't need to use a normal class Mnemonic that internally just uses MnemonicHelper from Apollo and I want to remove it.

Fixes #

Checklist

My PR contains...

• No code changes (changes to documentation, CI, metadata, etc.)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes
• are not breaking changes
• If yes to above: I have updated the documentation accordingly

Documentation

• My changes do not require a change to the project documentation
• My changes require a change to the project documentation
• If yes to above: I have updated the documentation accordingly

Tests

• My changes can not or do not need to be tested
• My changes can and should be tested by unit and/or integration tests
• If yes to above: I have added tests to cover my changes
• If yes to above: I have taken care to cover edge cases in my tests

[atala-dev]

Code Coverage

File	Coverage [57.14%]
atala-prism-sdk/src/commonMain/kotlin/io/iohk/atala/prism/walletsdk/apollo/ApolloImpl.kt	57.14%

Total Project Coverage	26.22%

[github-actions]

Unit Test Results

66 tests  ±0   62 ✔️ ±0   2s ⏱️ ±0s
19 suites ±0     4 💤 ±0 
19 files   ±0     0 ❌ ±0 

Results for commit 0621863. ± Comparison against base commit 0f487bd.