Rework the new version notice

[weppos]

The new release notice now prints a coloured headline with the version transition, the upgrade command for the detected install method, and the release page URL, with a blank line above and below. The URL prints for every install method, which replaces the Visit .../latest to upgrade. line that only appeared when the install method was unknown.

--no-color and the NO_COLOR environment variable turn the colour off. This is the first code that reads the --no-color flag; the NoColor field on output.Printer stays unused.

DefaultReleaseURL now names dnsimple/cli. #69 removed the dnsimple/homebrew-tap release mirror that the constant pointed at, so both the latest version lookup and the release page URL named a repository that no longer holds the artifacts.

🔍 QA

Scenario: the notice appears

• Build a binary with a version older than the latest release: go build -ldflags "-X main.version=0.1.0" -o /tmp/dnsimple ./cmd/dnsimple.
• Delete the cached state at state.json in the config directory, then run /tmp/dnsimple accounts list in a terminal.
• Confirm the block appears after the command output, with a blank line above it and below it. The headline is yellow, both version numbers are cyan, and the release URL is yellow.
• Confirm the release URL names dnsimple/cli and opens the release page for the new version.

Scenario: the upgrade command matches the installation

• Run the same binary from a Homebrew path, from ~/.dnsimple/bin, and from ~/go/bin.
• Confirm each run names the upgrade command for that installation method.
• Confirm a binary in an unrecognised path prints the headline and the URL, and omits the To upgrade, run: line.

Scenario: the notice is suppressed

• Run with --no-color, and again with NO_COLOR=1. Confirm the block appears with no colour.
• Run with DNSIMPLE_NO_UPDATE_CHECK=1. Confirm no block appears.
• Redirect the standard error stream to a file. Confirm no block appears.

📋 Deployment Pre/Post tasks

N/A

Deployment Verification

N/A