[receiver/awsecscontainermetricsreceiver] Remove v1.21.0 import

[anarwal]

Description

Removed v1.21.0 semconv import. Added feature gate receiver.awsecscontainermetrics.EmitContainerImageTagsAsSlice (alpha, disabled by default) to migrate from container.image.tag (string, deprecated) to container.image.tags (string array, semconv v1.40.0). No behavior change unless gate is explicitly enabled.

Link to tracking issue

Fixes #22095

Testing

Ran

go generate ./... && go mod tidy && go build ./... && go test ./...

from receiver/awsecscontainermetricsreceiver

Authorship

• I, a human, wrote this pull request description myself.

[atoulme]

Please resolve conflicts.

[anarwal]

@atoulme good to go now before something else breaks it :)