Remove all mentions of serverless plus

[shainaraskas]

Summary

We are going in a different direction for pricing for these features, so I am:

• removing mentions to the serverless plus add-on
• indicating that, for obs and sec serverless projects, private connectivity and ip filtering is gated by tier

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes validated cleanup is complete
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: cursor auto

[github-actions]

🔍 Preview links for changed docs

• deploy-manage/cloud-organization/billing/elasticsearch-billing-dimensions.md
• deploy-manage/cloud-organization/billing/serverless-project-billing-dimensions.md
• deploy-manage/deploy/elastic-cloud/differences-from-other-elasticsearch-offerings.md
• deploy-manage/deploy/elastic-cloud/project-settings.md
• deploy-manage/security/ip-filtering-cloud.md
• deploy-manage/security/ip-filtering.md
• deploy-manage/security/network-security-api.md
• deploy-manage/security/private-connectivity-aws.md
• deploy-manage/security/private-connectivity.md
• solutions/elasticsearch-solution-project/es-serverless-add-ons.md

More links …

• solutions/observability/observability-serverless-feature-tiers.md
• solutions/security/security-serverless-feature-tiers.md

[github-actions]

✅ Vale Linting Results

No issues found on modified lines!

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[marciw]

LGTM / TY 🚤

[florent-leborgne]

LGTM for experience-docs files

[alxchalkias]

LGTM!