Avoid linking a specific Java distribution (Adoptium/Temurin) in install + docs

[nthmost-orkes]

Summary

cli/install.sh and agentspan doctor both link users to https://adoptium.net/ when Java is missing or too old, and the install script defaults to brew install --cask temurin@21. Recommending a specific Java vendor in install paths and error messages has a couple of downsides:

• It implies endorsement and excludes equally valid choices (brew install openjdk@21, Amazon Corretto, Azul Zulu, system packages on Linux, Microsoft Build of OpenJDK, etc.).
• It goes stale: vendor URLs and download flow conventions change. Adoptium's Temurin landing changed structure several times in the last two years.
• It steers users toward installing yet another JDK when they may already have one their distro/IT prefers.

Suggested change

In cli/install.sh and agentspan doctor:

• Say "Java 21 or later" without naming a vendor.
• For auto-install (which is reasonable to keep on macOS/Linux), keep the current behavior but soften the messaging — something like "Installing Java 21 via Homebrew Temurin cask; you can also use any OpenJDK 21+ distribution."
• In the "not found" error messaging, replace the https://adoptium.net/... link with something like "Install Java 21 or later — any distribution works (Homebrew openjdk@21, Adoptium, Corretto, Zulu, …)."

Environment

• N/A — applies to all platforms.

Note

I file this one with the caveat that it's stylistic and opinions vary. Feel free to close as wontfix. It came up because we follow this rule on a separate project I work on and it has aged well there.