Downgrade polkadot to v6.8.1

[ghost]

Context (issues, jira)

Upgrading polkadot to v6.9.2 makes it impossible to build LLD and LLM, see https://ledgerhq.atlassian.net/browse/LL-8290.

Temporarily downgrade to v6.8.1 to unblock build of LLD and LLM.

Description / Usage

Expectations

• Test coverage: The changes of this PR are covered by test. Unit test were added with mocks when depending on a backend/device.
• No impact: The changes of this PR have ZERO impact on the userland. Meaning, we can use these changes without modifying LLD/LLM at all. It will be a "noop" and the maintainers will be able to bump it without changing anything.

[vercel]

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[codecov]

Codecov Report

Merging #1537 (5250b2d) into master (a1c5214) will decrease coverage by 13.36%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #1537       +/-   ##
===========================================
- Coverage   68.39%   55.02%   -13.37%     
===========================================
  Files         535      504       -31     
  Lines       22268    20289     -1979     
  Branches     5377     5015      -362     
===========================================
- Hits        15231    11165     -4066     
- Misses       6943     9079     +2136     
+ Partials       94       45       -49     

Impacted Files	Coverage Δ
src/libcore/createAccountFromDevice.ts	8.82% <0.00%> (-91.18%)	⬇️
src/families/elrond/api/apiCalls.ts	14.28% <0.00%> (-82.86%)	⬇️
src/families/algorand/libcore-buildSubAccounts.ts	16.36% <0.00%> (-81.82%)	⬇️
src/families/algorand/libcore-buildOperation.ts	15.62% <0.00%> (-81.25%)	⬇️
src/families/cosmos/libcore-buildOperation.ts	4.83% <0.00%> (-80.65%)	⬇️
src/libcore/buildAccount/index.ts	14.58% <0.00%> (-80.21%)	⬇️
src/families/cosmos/libcore-mergeOperations.ts	13.63% <0.00%> (-77.28%)	⬇️
src/families/bitcoin/js-signOperation.ts	21.87% <0.00%> (-76.57%)	⬇️
src/families/crypto_org/api/sdk.ts	16.85% <0.00%> (-75.29%)	⬇️
src/families/elrond/api/sdk.ts	23.07% <0.00%> (-75.00%)	⬇️
... and 167 more

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