Merge #17635: ci: Add CentOS 7 build

711e0449cf4a0f15cabe0d64094e3add24ad44b0 ci: Remove trusty build (Hennadii Stepanov)
7f3ae224685efaeb6fe714de90e8871d12e55f34 ci: Add CentOS 7 build (Hennadii Stepanov)

Pull request description:

  Arguably, CentOS is the most conservative distro of all the popular ones. Thus, it could be a good way to check the Bitcoin Core compatibility with aged dependencies.

  Currently, CentOS 7 has:
  - Berkeley DB == 4.8.30
  - Boost == 1.53.0
  - GCC == 4.8.5
  - libevent == 2.0.21 < minimum required [2.0.22](https://github.com/bitcoin/bitcoin/blob/master/doc/dependencies.md), but tests passed
  - MiniUPnPc == 2.0
  - Python == 3.6.8
  - qrencode == 3.4.1
  - Qt == 5.9.7
  - ZeroMQ == 4.1.4

  ~Please note that this PR is based on the bugfix #17634.~

  Also trusty build has been removed for the following reasons:
  - https://github.com/bitcoin/bitcoin/issues/17628#issuecomment-559448201:
  > Maybe it'd make sense to replace Ubuntu Trusty with Centos 7 as the "check ancient backward compatibililty" Travis run. It's supported until 2024, apparently.

  - https://github.com/bitcoin/bitcoin/pull/17635#discussion_r354811792:
  > Our travis is currently running at its limit and this doesn't seem like it is adding a lot new coverage compared to the other builds.

  Close #17628

ACKs for top commit:
  MarcoFalke:
    ACK 711e0449cf4a0f15cabe0d64094e3add24ad44b0 🚠

Tree-SHA512: 614ec8394943f482a5867067f7119bffd052924a51e32ffda9a08e10c392c4a955a3539e2f8907cb65bfd9347dadf0ba62f6d1530bbc49927c347360a5a7f73c

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