feat: Resolve "Solutions Startups Join related to [ENG] /solutions/*4 pages migration"

Step 1: What is changing in this MR?

• Uses $solutionsStartupsService for data fetching and transformation
• Transforms and expands $solutionsStartupsService to suit /startups/join/ page
• Adjusts usage of $solutionsStartupsService in /startups/ page
• Makes mapFaq method from $freeTrialService public and uses it in $solutionsStartupsService
• Deletes all /content/solutions/startups/*.yml files in this repo
• Adds pages with deleted content to route.contentful.js

Review app	Prod
Startups join	Startups Join Prod
Startups landing page	Startups landing page Prod

Step 2: Ensure that your changes comply with the following, where applicable:

• I, the Assignee, have run Axe tools on any updated pages, and fixed the relevant accessibility issues.
• These changes meet a specific OKR or item in our Quarterly Plan.
• These changes work on both Safari, Chrome, and Firefox.
• These changes have been reviewed for Visual Quality Assurance and Functional Quality Assurance on Mobile, Desktop, and Tablet.
• These changes work with our Google Analytics and SEO tools.
• These changes have been documented as expected.

Step 3: Add the appropriate labels for triage

This MR will have dex-approval::2-standard automatically applied, but please update it as follows. If deciding between two levels, go with the higher of the two: https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/sites/handbook/source/handbook/marketing/digital-experience/marketing-site-approval-process/index.html.md

Step 4: Tag the appropriate person for review

Depending on which label is used, you may tag the following people as a Reviewer on this MR: https://about.gitlab.com/handbook/marketing/digital-experience/marketing-site-approval-process/#step-3-tag-the-appropriate-people-for-review

Related to #2924 (closed)