chore: replace volta with mise for toolchain management (#2143)

chore: replace volta with mise for toolchain management (#2143)

* chore: replace volta with mise for toolchain management

Volta is unmaintained and its maintainers recommend mise. Declare node and
pnpm in .mise.toml, drop the per-package volta blocks, point the sea script at
mise which node and update the README setup instructions.

CI is unchanged: it already provisions node and pnpm through
actions/setup-node and pnpm/action-setup, and its node matrix must keep testing
several versions.

Signed-off-by: Jérôme Benoit <jerome.benoit@sap.com>
* chore: read mise toolchain versions from package.json

Follow poolifier#3334 without duplicating Node.js or pnpm pins. Keep explicitly selected runtimes through onFail ignore, and require a mise version verified to install the current pnpm release assets. Correct the Corepack setup instructions and ignore personal mise overrides and lockfiles.

Signed-off-by: Jérôme Benoit <jerome.benoit@sap.com>
* docs: keep development setup instructions focused on the project

Signed-off-by: Jérôme Benoit <jerome.benoit@sap.com>
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Signed-off-by: Jérôme Benoit <jerome.benoit@sap.com>