MINIFICPP-952 remove travis and appveyor

[szaszm]

When we switched to GitHub Actions CI in #853, I wanted to keep Travis and Appveyor around for some time. The time has come to finalize the switch. This PR removes Travis and Appveyor, which are both less stable than GitHub Actions.

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Thank you for submitting a contribution to Apache NiFi - MiNiFi C++.

In order to streamline the review of the contribution we ask you
to ensure the following steps have been taken:

For all changes:

• Is there a JIRA ticket associated with this PR? Is it referenced
  in the commit message?

• Does your PR title start with MINIFICPP-XXXX where XXXX is the JIRA number you are trying to resolve? Pay particular attention to the hyphen "-" character.

• Has your PR been rebased against the latest commit within the target branch (typically main)?

• Is your initial contribution a single, squashed commit?

For code changes:

• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE file?
• If applicable, have you updated the NOTICE file?

For documentation related changes:

• Have you ensured that format looks appropriate for the output in which it is rendered?

Note:

Please ensure that once the PR is submitted, you check travis-ci for build issues and submit an update to your PR as soon as possible.

[hunyadi-dev]

This is great. Did you check other artifacts referring to these CI jobs too?

➜  git --no-pager grep --name-only travis | grep -v thirdparty
.github/PULL_REQUEST_TEMPLATE.md
README.md
bootstrap.sh
➜   git --no-pager grep --name-only appveyor | grep -v thirdparty
README.md
appveyor.yml

[hunyadi-dev]

Great. Bye-bye travis and appveyor.