Minor update: max elements in ActionsBlock to 25

[YSaxon]

It seems from practical testing with the slack block kit builder that the (new?) max number of elements allowed in an ActionsBlock is 25, rather than 5 as previously enforced in the code.

Category (place an x in each of the [ ])

• [ ] slack_sdk.web.WebClient (sync/async) (Web API client)
• [ ] slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• [ ] slack_sdk.socket_mode (Socket Mode client)
• [ ] slack_sdk.signature (Request Signature Verifier)
• [ ] slack_sdk.oauth (OAuth Flow Utilities)
• [X] slack_sdk.models (UI component builders)
• [ ] slack_sdk.scim (SCIM API client)
• [ ] slack_sdk.audit_logs (Audit Logs API client)
• [ ] slack_sdk.rtm_v2 (RTM client)
• [ ] /docs-src (Documents, have you run ./scripts/docs.sh?)
• [ ] /docs-src-v2 (Documents, have you run ./scripts/docs-v2.sh?)
• [ ] /tutorial (PythOnBoardingBot tutorial)
• [ ] tests/integration_tests (Automated tests for this library)

Requirements (place an x in each [ ])

• [X] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [X] I've read and agree to the Code of Conduct.

[CLAassistant]

All committers have signed the CLA.

[YSaxon]

(slack block kit builder showing max elements in an actionblock = 25)

[seratch]

Merged by https://github.com/slackapi/python-slack-sdk/commit/623ace0c18865eea4d60d9432df383eac835c047 - I've added a commit to apply corresponding changes to the existing unit tests. Thanks again fro your contribution!