Revert jenkins removal

[fearful-symmetry]

What does this PR do?

Partly reverts #3771

Turns out that some scripts in dev-tools still depends on scrips in .ci/, so we're breaking the unified release process.

This PR should be reverted once the dangling references to the files in .ci/ are dealt with:

➜  elastic-agent git:(revert-jenkins-removal) ✗ grep -rn "\.ci/" .
./.pre-commit-config.yaml:11:        files: ^(.ci/(.*\.groovy|Jenkinsfile)|Jenkinsfile)$
./magefile.go:452:// this follows the same logic as https://github.com/elastic/beats/blob/2fdefcfbc783eb4710acef07d0ff63863fa00974/.ci/scripts/prepare-release-manager.sh
./.github/workflows/bump-golang.yml:24:          pipeline: ./.ci/bump-golang.yml
./dev-tools/common.bash:48:  GO_VERSION="${version}" "${BASEDIR}"/.ci/scripts/install-go.sh

[mergify]

This pull request does not have a backport label. Could you fix it @fearful-symmetry? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[jlind23]

LGTM

[jlind23]

Force merging this to unblock the unified release process.

[elastic-sonarqube]

SonarQube Quality Gate

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