feat: 🎸 Drop v7 support

[prashantasdeveloper]

Description

Drops all v7 chain support/backward-compatibility code, now that the SDK only targets v8. Removes Context.isV7, all v7/v8 branching, and every feature that only existed for v7 chains.

Removed entities/features (v7-only, no v8 equivalent):

• ChildIdentity entity and all child-identity APIs: Identity.getChildIdentities, Identity.unlinkChild, Identity.isChild, createChildIdentity/createChildIdentities/unlinkChildIdentity procedures, Context.getChildIdentity.
• CDD claims: Claims.getCddClaims, Identity.hasValidCdd, CddClaim handling, RoleType.CddProvider / isCddProviderRole (superseded by DidRegistrar role, which is now used unconditionally instead of the v7 cddServiceProviders pallet).
• Legacy relayer-paying-key authorization flow: consumeAddRelayerPayingKeyAuthorization procedure and AuthorizationType.AddRelayerPayingKey (the OldAddRelayerPayingKey type/flow remains; accepting the removed type now throws NotSupported pointing users at AccountManagement.approveSubsidy).
• Instruction affirmation withdrawal: InstructionAffirmationOperation.Withdraw/WithdrawAsMediator and Instruction.withdraw/withdrawAsMediator (use reject/rejectAsMediator instead).
• Nft.controllerTransfer's deprecated destinationPortfolio param (use destination).
• AcceptPrimaryKeyRotationParams.cddAuth and the associated CDD-authorization validation/consumption in acceptPrimaryKeyRotation.
• SetStakingControllerParams — Staking.setController is now a no-arg procedure since v8 always makes the stash its own controller; likewise bondPolyx no longer accepts/sends a controller argument.

Simplified now-unconditional behavior (previously branched on context.isV7):

• Context.getAccountBalance only uses the v8 FrameSystemAccountInfo shape (AccountInfo/legacy assembly removed).
• transferPolyx always uses balances.transferWithMemo with an optional memo (v7's memo-less balances.transfer path removed).
• Instruction.generateOffChainAffirmationTransactionRaw: expiresAt is now always mandatory, and the payload always includes the v8 receipt format.
• Identity.checkRoleForDid/isGcMember always query didRegistrars.activeMembers (the v7 cddServiceProviders.activeMembers fallback is gone).
• SUPPORTED_SPEC_VERSION_RANGE narrowed from '7.0 || 7.1 || 7.2 || 7.3 || 8.0' to '8.0'.
• isV7Spec util removed along with Context.isV7.

Docs/typos: CreateMultiSigParams note updated from "Signer must be an Account as of v7" to "Signer must be an Account".

Breaking Changes

JIRA Link

Checklist

• Updated the Readme.md (if required) ?

[sonarqubecloud]

Quality Gate failed

Failed conditions
100.0% Coverage on New Code (required ≥ 100%)
D Reliability Rating on New Code (required ≥ A)
1 New Code Smells (required ≤ 0)

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