Clarify Dependabot default ecosystem label behavior and naming

[alok-108]

Why:

Closes: #46230

Resolves contradictions between dependabot-options-reference.md and data/reusables/dependabot/default-labels.md:

1. Dependabot applies both the dependencies label and the ecosystem label to all pull requests (including single-ecosystem updates), not just when more than one package manager is defined.
2. The default label created and applied for GitHub Actions is github_actions (with an underscore), not github-actions.

What's being changed:

• data/reusables/dependabot/default-labels.md: Fixed the example ecosystem label for GitHub Actions from github-actions to github_actions.
• content/code-security/reference/supply-chain-security/dependabot-options-reference.md: Updated the default behavior description under labels to state that the ecosystem label is added to all pull requests, including single-ecosystem updates (aligning with default-labels.md and observed Dependabot behavior), and referenced github_actions.

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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Table of review links

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
code-security/reference/supply-chain-security/dependabot-options-reference.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18
code-security/tutorials/secure-your-dependencies/customizing-dependabot-prs.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18	from reusable

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

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