Remove slack link in Japanese translation

[vyshnaviJammi]

🎯 Goal
Replace link to slack in 'SlackTeam' section in Japanese translation of Readme and put a link to code contributions like in English Readme.

💡 Possible solutions
Remove link to slack in Japanese translation and put a link to [code contributions repo] (https://github.com/jammivyshnavi/code-contributions) similar to main Readme.
You can find it in https://github.com/firstcontributions/first-contributions/blob/main/docs/translations/README.ja.md

Important

Please only address one issue in a PR.
Let's make it possible for more people to solve issues here.

📋 Steps to solve the problem

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[Yojitkataria]

"Hey! I'd like to work on this issue 🚀"

[nadhamsaipallavi]

I have removed the slack line can you review my pr #96566