Update Welcome Bot messages

[alya]

As a follow-up to #29298, we should update the Welcome Bot's initial DM message by replacing the first part.

Old:

Hello, and welcome to Zulip!:wave: This is a direct message from me, Welcome Bot.

If you are new to Zulip, check out our [Getting started guide](/help/getting-started-with-zulip)! We also have a guide for [Setting up your organization](/help/getting-your-organization-started-with-zulip).

New:

Hello, and welcome to Zulip! 👋 I've kicked off some conversations to help you get started. You can find them in your [Inbox](#inbox). 

To learn more, check out our [Getting started guide](/help/getting-started-with-zulip)! We also have a guide for [Setting up your organization](/help/getting-your-organization-started-with-zulip).

Notes:

• The same logic for admin vs. non-admin content still applies.

Blockers:

• Mark onboarding messages as unread and as starred #29298

[zulipbot]

Hello @zulip/server-bots, @zulip/server-onboarding members, this issue was labeled with the "area: onboarding", "area: bots" labels, so you may want to check it out!

[dheerajalim]

@zulipbot claim

[zulipbot]

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