CI housekeeping

[kianmeng]

This PR ensures all CI jobs pass without errors.

List of changes:

• add support for OTP26/27 in CI
• bump actions/checkout
• drop support for OTP19 (not supported in official Docker image) from CI
• drop support for OTP20 (not supported by rebar3_hex) from CI
• fix test errors due to updated certs
• remove Travis CI config
• resolve dialyzer error of unknown function public_key:pem_decode/1 in OTP26/27

[kianmeng]

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