Update supported Terraform version in Dependabot to 1.15.x

[KazuyaMiyagi]

Why:

Follow-up to dependabot/dependabot-core#15694, which bumps the Terraform version bundled in the Dependabot updater image from 1.15.3 to 1.15.8. CONTRIBUTING.md in dependabot-core asks contributors to file a corresponding docs PR when bumping a bundled version.

Opened as a draft: the version documented here is not shipped until dependabot/dependabot-core#15694 is merged and rolled out. I'll mark this ready for review once that happens.

While preparing this I noticed the supported Terraform version is documented in two places that have drifted apart:

File	Currently says
content/code-security/reference/supply-chain-security/dependabot-options-reference.md	>= 0.13, <= 1.10.x
data/reusables/dependabot/supported-package-managers.md	>= 0.13, <= 1.13.x

The bundled version was last bumped to 1.13.3 in September 2025, and the reusable was updated accordingly in #40672. The copy in dependabot-options-reference.md was introduced in February 2025 by the PR that addressed the disparity between the Dependabot docs articles, carrying the then-current 1.10.x, and has not been updated since. That file already carries a note asking for both copies to be kept in sync:

<!--Note: When making updates to this section, please make sure any changes are also reflected in `data/reusables/dependabot/supported-package-managers.md`.-->

This PR brings both to the same value.

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

One line in each of the two files, so the supported Terraform version reads >= 0.13, <= 1.15.x in both:

-| Terraform    | `terraform`      | >= 0.13, <= 1.10.x  |
+| Terraform    | `terraform`      | >= 0.13, <= 1.15.x  |

-[Terraform](#terraform)      | `terraform`      | >= 0.13, <= 1.13.x  | ...
+[Terraform](#terraform)      | `terraform`      | >= 0.13, <= 1.15.x  | ...

The .x notation follows the four previous updates to this value (#18646, #23298, #35614, #40672).

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 value depends on build(deps): bump terraform to 1.15.8 dependabot/dependabot-core#15694, so an SME review from GitHub staff is likely needed
• 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. — 42 checks are green; the 5 skipped ones are GitHub-internal workflows (repo-sync, team membership, legal reviewers). As an outside contributor I can't see the review environment — the bot notes a Hubber has to deploy it internally — so I haven't viewed it; the change is a single table cell in each file

[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@ 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17
code-security/reference/supply-chain-security/supported-ecosystems-and-repositories.md	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17	from reusable

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

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