Stopped the generated Armv8-M readmes claiming a false origin date (#623)

Every Armv8-M port readme ends with

    09-30-2020  Initial ThreadX 6.1 version for Cortex-M85 using GNU tools.

with the core name substituted in by scripts/copy_armv8_m.sh. The date is the
shared port's, so each core inherits it whatever its own history: Cortex-M85 was
announced in 2022 and its readme claims a 2020 origin, and any core added later
gets the same treatment the moment its name joins the generator's list.

The rest of the history block is accurate, since it records changes to the shared
files. Only the closing line asserts something per-core. Reword it to describe
the Armv8-M port itself, and say where a given core's real starting point is.

Regenerating updates the twelve readmes for cortex_m33, cortex_m52, cortex_m55
and cortex_m85 across the three toolchains.

Cortex-M52 makes the point: it arrived in #519 and its readme immediately claimed
a 2020 origin for a core announced in 2023.

The ARMv7-M templates say "Initial ThreadX version 6.1.7 for Cortex-M", with no
placeholder to substitute, so they make no per-core claim and are left alone.

Assisted-by: Claude Code (Opus 5) <[email protected]>

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