docs(dependabot): distinguish private repositories from registries

[aniruddhaadak80]

Why:

Closes: #45681

The Dependabot support matrix has separate Private repositories and Private registries columns, but the article does not explain what each column represents. The surrounding guidance explains how to grant access to private repositories and configure private registries, but readers can still interpret “Private repositories” as the repository being updated rather than a repository that hosts dependencies. This distinction is important because support for one hosting type does not imply support for the other.

What's being changed (if available, include any code snippets, screenshots, or gifs):

• Rename the matrix column to Private Git repositories so it is clear that it refers to repositories hosting dependencies.
• Add a short explanation distinguishing private Git repositories from private registries and identifying both as forms of private dependencies.
• Leave all ecosystem support values unchanged.
• Evidence: the current article and its support matrix are at https://docs.github.com/en/code-security/reference/supply-chain-security/supported-ecosystems-and-repositories, and the existing guidance describes private repository access separately from registries configuration.
• Testing:
  • npm run build — passed.
  • npm run lint-content -- --paths content/code-security/reference/supply-chain-security/supported-ecosystems-and-repositories.md data/reusables/dependabot/supported-package-managers.md — passed with no errors.
  • npm test -- src/content-linter/tests/lint-files.ts src/content-linter/tests/unit/table-column-integrity-simple.ts — 3469 tests passed.
  • npm test -- src/content-render/tests/render-changed-and-deleted-files.ts — 2 tests passed.
  • git diff --check — passed.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Source	Review	Production	What Changed
code-security/reference/supply-chain-security/supported-ecosystems-and-repositories.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[subatoi]

Hi, regrettably I'm going to close this out—we haven't marked this as ready for community contributions yet, and are yet to make a determination about the best way to tackle the problem. Apologies that this won't be the outcome you were hoping for