CDTOOL-1235: Provide links to major version release notes in 'new version available' output.

[kpfleming]

Change summary

When the CLI informs the user that a newer version is available for installation, it will now detect whether that new version is at least one major version higher than the installed version. If it is, it will provide the user links to the changelog (release notes) for each of the major versions between the current version and the latest version. There is no change in the output if the current version and latest version have the same major version number.

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Sample output (one major version behind):

kpfleming@kpf25:~/src/fastly/cli$ ./fastly version
Fastly CLI version v12.1.0 (unknown)
Built with go version go1.25.5 linux/amd64 (2026-01-16)

A new version of the Fastly CLI is available.
Current version: 12.1.0
Latest version: 13.3.0

Note: Please review the release notes for the major version(s) listed below before upgrading.
Version 13.0.0: https://github.com/fastly/cli/releases/tag/v13.0.0
Run `fastly update` to get the latest version.

---

Sample output (multiple major versions behind):

kpfleming@kpf25:~/src/fastly/cli$ ./fastly version
Fastly CLI version v10.1.0 (unknown)
Built with go version go1.25.5 linux/amd64 (2026-01-16)

A new version of the Fastly CLI is available.
Current version: 10.1.0
Latest version: 13.3.0

Note: Please review the release notes for the major versions listed below before upgrading.
Version 11.0.0: https://github.com/fastly/cli/releases/tag/v11.0.0
Version 12.0.0: https://github.com/fastly/cli/releases/tag/v12.0.0
Version 13.0.0: https://github.com/fastly/cli/releases/tag/v13.0.0
Run `fastly update` to get the latest version.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

• Does your submission pass tests?

Changes to Core Features:

• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

[rcaril]

LGTM