release v0.5.0; ensure "latest" Docker tag is pushed; use versioned Docker tag in the Chart

[trentm]

• Bump the version to v0.5.0 for a release.
• Change the chart values to use the matching "docker.elastic.co/observability/apm-attacher:v$VERSION" tagged Docker image instead of using the "latest".
• Add a lint step to ensure those versions are in sync.
• Drop the "publish" CI workflow that would tag/push a new Docker image for every commit to main. These aren't used or needed.
• Bump the version of some "actions/..." in CI workflows.
• Also added a RELEASING.md to document the release process.

Fixes: https://github.com/elastic/apm-k8s-attacher/issues/42 Fixes: https://github.com/elastic/apm-k8s-attacher/issues/95

[trentm]

Tagged. Release CI run: https://github.com/elastic/apm-k8s-attacher/actions/runs/7532271129

[trentm]

The v0.5.0 and latest docker tags are the same image:

% docker pull docker.elastic.co/observability/apm-attacher:v0.5.0
v0.5.0: Pulling from observability/apm-attacher
661ff4d9561e: Already exists
fbafe6599ec7: Pull complete
2f9716fad2d8: Pull complete
a9116d0b4538: Pull complete
Digest: sha256:72a62ef042c11bf9a7c7853fc4353159019aedb49880992f962ce3c3d2e140c2
Status: Downloaded newer image for docker.elastic.co/observability/apm-attacher:v0.5.0
docker.elastic.co/observability/apm-attacher:v0.5.0

% docker pull docker.elastic.co/observability/apm-attacher:latest
latest: Pulling from observability/apm-attacher
Digest: sha256:72a62ef042c11bf9a7c7853fc4353159019aedb49880992f962ce3c3d2e140c2
Status: Downloaded newer image for docker.elastic.co/observability/apm-attacher:latest
docker.elastic.co/observability/apm-attacher:latest

[trentm]

Helm caches have updated:

% helm search repo elastic/apm-attacher
NAME                    CHART VERSION   APP VERSION DESCRIPTION
elastic/apm-attacher    0.5.0                       A Helm chart installing the Elastic APM Kuberne...