Compilation errors with TypeScript v4.4 in CI builds

[seratch]

Description

src/App.ts:680:7 - error TS2571: Object is of type 'unknown'.

680       error.code = 'slack_bolt_authorization_error';
          ~~~~~

src/App.ts:681:31 - error TS2345: Argument of type 'unknown' is not assignable to parameter of type 'Error'.

681       return this.handleError(error);
                                  ~~~~~

src/App.ts:857:31 - error TS2345: Argument of type 'unknown' is not assignable to parameter of type 'Error'.

857       return this.handleError(error);
                                  ~~~~~

src/conversation-store.ts:67:13 - error TS2571: Object is of type 'unknown'.

67         if (error.message !== undefined && error.message !== 'Conversation not found') {
               ~~~~~

src/conversation-store.ts:67:44 - error TS2571: Object is of type 'unknown'.

67         if (error.message !== undefined && error.message !== 'Conversation not found') {
                                              ~~~~~

src/conversation-store.ts:69:97 - error TS2571: Object is of type 'unknown'.

69           logger.debug(`Conversation context failed loading for ID: ${conversationId}, error: ${error.message}`);
                                                                                                   ~~~~~

src/receivers/HTTPReceiver.ts:169:13 - error TS2571: Object is of type 'unknown'.

169         if (error.code === ErrorCode.HTTPReceiverDeferredRequestError) {
                ~~~~~

src/receivers/HTTPReceiver.ts:279:58 - error TS2571: Object is of type 'unknown'.

279         this.logger.warn(`Request verification failed: ${err.message}`);
                                                             ~~~

src/receivers/HTTPReceiver.ts:292:53 - error TS2571: Object is of type 'unknown'.

292         this.logger.warn(`Malformed request body: ${err.message}`);
                                                        ~~~

src/receivers/SocketModeReceiver.ts:126:29 - error TS2345: Argument of type 'unknown' is not assignable to parameter of type 'string | undefined'.
  Type 'unknown' is not assignable to type 'string'.

126             throw new Error(err);
                                ~~~

Found 10 errors.

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• enhancement (feature request)
• question
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Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.