bootstrap: Set ECDSA as the default SSH key algorithm

Motivation: RSA SHA-1 SSH keys are no longer accepted by GitHub https://github.blog/2021-09-01-improving-git-protocol-security-github/

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Given this we are switching the default from RSA to ECDSA for `git`, `github` and `gitlab` variants of `flux bootstrap`.

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>