GitHub Actions deprecations part II

[spaze]

In #37/#39 I've upgraded some GitHub Actions to not throw deprecation warnings but some actions are still using those deprecated commands and runtimes:

• mbogh/test-ssl-action@v2 uses node12 runtime, once upgraded to use node16 the deprecation warning should disappear
  Use Node.js 16 mbogh/test-ssl-action#3 Node12 is deprecated mbogh/test-ssl-action#4
  Done in Use the new testssl action version to support Node16 #198

• symfonycorp/security-checker-action@v4 uses set-output
  Done in Bump symfonycorp/security-checker-action from 4 to 5 #231

  • Getting a deprecation warning about this step in the action in V3 / V4  symfonycorp/security-checker-action#8
  • GitHub actions deprecating set-output command fabpot/local-php-security-checker#67
    Both issues closed now but not the fix hasn't been released yet
    Now released, see Getting a deprecation warning about this step in the action in V3 / V4  symfonycorp/security-checker-action#8 (comment)
  • Update README to use v5 symfonycorp/security-checker-action#10

These should be upgraded too, once their new versions become available.

[spaze]

Done in #198 & #231