fix(guides/spring): configure plaintext for spring SDK

[meyerdan]

The spring boot getting started guide uses c8run.
C8run does not use ssl / tls
Adding the client config to make sure that the SDK can authenticate.

fixes to #4911

Description

Should be backported to 8.6

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support. (add bug or support label)
• This is already available but undocumented and should be released within a week. (add available & undocumented label)
• This is on a specific schedule and the assignee will coordinate a release with the DevEx team. (create draft PR and/or add hold label)
• This is part of a scheduled alpha or minor. (add alpha or minor label)
• There is no urgency with this change (add low prio label)

PR Checklist

• My changes are for the next minor and are in /docs directory (aka /next/).
• My changes are for an already released minor and are in /versioned_docs directory.

• I added a DRI, team, or delegate as a reviewer for technical accuracy and grammar/style:
  • Engineering team review
  • Technical writer review via @camunda/tech-writers unless working with an embedded writer.

[github-actions]

👋 🤖 ✅ Looks like the changes were ported across versions, nice job! 🎉

You can read more about the versioning within our docs in our documentation guidelines.

[akeller]

@pepopowitz can you please confirm this is correct by running the guide? We should get this merged and released ASAP if it works as the guide currently doesn't work at all.

[pepopowitz]

@pepopowitz can you please confirm this is correct by running the guide? We should get this merged and released ASAP if it works as the guide currently doesn't work at all.

I've asked for a substitute reviewer in our team channel. This is a non-trivial review, and I am desperate to complete my own project this week before cutting a new release makes it harder for me.

Though TBH if @meyerdan said that he confirmed these settings in action, I'd trust his word and merge it, and review it when I have more time next week.

[conceptualshark]

I ran these changes against 8.7.0-alpha3 and 8.6.7 for both the SDK and Camunda Run, and was able to complete the guide in both instances. However, I was also able to complete the guide without the added lines. I'm not terribly familiar with the SDK or its setup, so not sure if that is error on my part, or would be better tested again. I wonder if @jessesimpson36 has any C8Run auth insights? Not sure why it would run fine for me.

However, I went ahead and backported these changes to 8.6 as requested, to speed up approving and merging.

Separately, I ran into a few issues with the 8.7 guide as a whole, and created #4925 to make sure those get addressed.

[akeller]

I ran these changes against 8.7.0-alpha3 and 8.6.7 for both the SDK and Camunda Run

@meyerdan, what version were you running? Latest for everything? @megglos any thoughts about why this worked for Cole but not Daniel?