Deprecate tap formulas now that PowerShell is in Homebrew Core (#1348)

* Deprecate tap formulas now that PowerShell is in Homebrew Core

PowerShell is now published to Homebrew Core as the community-maintained
`powershell` formula (Homebrew/homebrew-core#268901), so the Homebrew
project itself maintains a formula in our place. We were subsumed, but in
a good way: there's no reason to keep pointing people at this broken,
unmaintained tap.

Add `deprecate!` to the three still-active formulas so anyone installing
via `powershell/tap/*` gets a warning steering them to `brew install
powershell`:

- `powershell`, `powershell-lts`, and `powershell-preview` get
  `deprecate! date: "2026-06-18", because: "is now maintained by Homebrew
  Core as the `powershell` formula"`
- `powershell-daily` is left alone since it's already `disable!`d, which
  supersedes `deprecate!`

I chose `deprecate!` over `disable!` so `brew install` still works (just
warns) and the install-and-test CI keeps passing. The custom `because:`
string satisfies Homebrew's `DeprecateDisableReason` cop (no leading `it`,
no trailing punctuation) and is verified with `brew style`.

Follow-up to #1347, which added the README deprecation notice.

Drafted by Copilot (Claude Opus 4.8).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Remove formula CI workflows ahead of archiving the tap

The `powershell`, `powershell-lts`, and `powershell-preview` install/test
jobs were already failing: the formulas no longer build now that
`depends_on macos: :high_sierra` is disabled upstream (#1322, #1339,
#1341), and we just deprecated them in favor of Homebrew Core. There's
nothing left worth testing or auto-updating before the repo is archived,
so drop the formula CI rather than chase green checks.

- Delete the `PR-powershell{,-lts,-preview}.yml` pull-request workflows
- Delete the `cron-powershell{,-preview}.yml` auto-update workflows
- Delete the `composite/{prep,installAndTest,createPR}` actions they used

`scorecard.yml` is left in place since it's unrelated to formula CI and
still passes.

Drafted by Copilot (Claude Opus 4.8).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Reword LTS and preview deprecation to not imply a Core replacement

Homebrew Core publishes only the stable `powershell` formula; it has no
LTS or preview equivalent. The previous `because:` text claimed these
formulas are "now maintained by Homebrew Core as the `powershell`
formula," which misleads `powershell-lts` and `powershell-preview` users
into expecting a matching Core formula that does not exist.

State instead that the tap is unmaintained now that PowerShell ships in
Core, and direct users in the comment to the stable formula or an
official PowerShell package. The stable `powershell.rb` wording is
accurate and is left unchanged.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: andyleejordan