feat!: Raise the consumer psake floor to 5.0.4

[tablackburn]

Closes #166. Part of #120 (Phase 3).

Breaking. RequiredModules now requires psake 5.0.4 or newer, previously 4.9.0. psake
4.x is no longer supported. Invoke-Build users are unaffected.

Why — the argument that holds

The floors this module declares were not all earned the same way:

Dependency	Floor	What CI actually tests
BuildHelpers	2.0.16	2.0.16
Pester	5.6.1	6.0.0 and 5.9.0 — both majors, via the #137 matrix
psake	4.9.0 → 5.0.4	5.0.4 only

Pester's lower floor is earned — Test-PSBuildPester supports both majors and CI proves it on
every run. The psake floor was lower and untested: CI has exercised only 5.0.4 since #162 moved
the toolchain, so 4.9.0 asserted support that nothing verified. #120 already flagged exactly this
under Not yet specified.

That left two honest options — add a psake 4.x CI leg, or stop claiming 4.x. For a volunteer
project, maintaining a second task-runner matrix leg for a major nobody has asked for is the more
expensive one, and 1.0.0 is the cheapest moment to move a floor.

The principle is do not claim support you do not test — not "everything on latest", which would
also have raised the Pester floor that #120 deliberately kept low.

Two arguments deliberately not relied on

Recorded so this is not re-litigated on grounds that do not survive checking:

• "One supported major simplifies psakeFile.ps1." Not today. There is no version-conditional
  psake code anywhere — IB.tasks.ps1 does not contain the string "psake", and psakeFile.ps1
  touches only $psake.context.currentTaskName and $psake.context.Peek().Tasks.Keys, both present
  in 4.9.x and 5.0.4 alike. The simplification is prospective.
• "Nobody runs anything older than PowerShell 5.x." True, and already enforced by
  PowerShellVersion = '5.1' from chore: Raise minimum PowerShell version to 5.1 in module manifest #141. psake 4.9.1 runs perfectly well on PowerShell 7, so a
  consumer can be entirely current and still be on psake 4.

Known impact

RequiredModules is enforced at import, so this is not a graceful degradation — with only psake 4.x
installed, Import-Module PowerShellBuild fails outright.

One concrete consumer worth notifying rather than surprising: devblackops/Terminal-Icons pins
psake = '4.9.0' in its requirements.psd1 and uses PowerShellBuild. Found by searching real
requirements.psd1 files rather than assuming.

What it costs them, per psake's own migration guide: most v4 build scripts work unchanged. The
Task ... -Depends syntax, -FromModule, and $psake.build_success are all explicitly retained —
this repository is the proof, since it still uses all three under 5.0.4.

Also fixed here

README.md claimed psake 4.8.0 was required and showed
Install-Module -Name psake -RequiredVersion 4.8.0, while the manifest floor had been 4.9.0.
Wrong before this change and more wrong after it, so it is corrected in the same pass — including
switching -RequiredVersion to -MinimumVersion, since the floor is a minimum.

Verification

• Test-ModuleManifest validates against the raised floor
• Full suite: 472 passed, 0 failed
• Migration-guide entry added with the full list of psake v4→v5 breaks and what each costs a
  consumer, plus the one thing they gain: psake 5 stops silently swallowing an escaping break,
  which under 4.9.x hid twelve non-running tests in this repository (psake 5.x spike: assess breakage under psake 5.0.4 (clean bump, no extras) #155)

The second commit repairs doubled carriage returns my own edit introduced in CHANGELOG.md — a
nine-line addition was showing as a 736-line diff. Same mistake as 7db09e7, same cause.

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