chore: avoid deprecated funcs to calculate sha256

[federico-sysdig]

What type of PR is this?
/kind cleanup

Any specific area of the project related to this PR?
/area build

What this PR does / why we need it:
When building Falco with recent versions of OpenSSL all SHA256_* functions are reported as being deprecated. This PR uses a set of different functions to calculate a SHA256 checksum that avoids the deprecation. The behavior is the same.

Which issue(s) this PR fixes:
None

Special notes for your reviewer:

Does this PR introduce a user-facing change?:
No

[poiana]

Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[github-actions]

This PR may bring feature or behavior changes in the Falco engine and may require the engine version to be bumped.

Please double check userspace/engine/falco_engine_version.h file. See versioning for FALCO_ENGINE_VERSION.

/hold

[federico-sysdig]

/unhold

[FedeDP]

/approve

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: FedeDP, federico-sysdig

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[poiana]

LGTM label has been added.

Git tree hash: bea2aaab74499f092088b4e2bf3cee121b7ba1b7

[FedeDP]

/milestone 0.40.0

[federico-sysdig]

@incertum, may I ask you to double-check on this PR?
It's quick and I'd like to be sure that my desire to avoid functions that are deprecated in recent versions of OpenSSL is not causing disruption.