**What this PR does / why we need it**:

Rename `gel-license.jwt` property of secret `gel-secrets` to
`license.jwt` on enterprise-logs chart to make it consistent betwen
`gel-secrets` and `gel-license` kubectl secrets.

* Fix the `stringData` property of the GEL secret, to contain
`license.jwt` property when importing:
* `gel-secrets`: containing both the `gcp_service_account.json` and
`license.jwt` properties
https://github.com/grafana/loki/blob/6ece17a82f15319b58befaef9aefcd9059ad266d/production/helm/loki/docs/examples/enterprise/enterprise-secrets.yaml#L1-L12
* `gel-license`: containing only the `license.jwt` property
https://github.com/grafana/loki/blob/6a7403c4f5cd0a01dea13954fe6f51631f485ad1/tools/dev/k3d/environments/helm-cluster/main.jsonnet#L213-L217
* Minor fixes in the README documentation.

**Special notes for your reviewer**:

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [ ] Tests updated
- [x] `CHANGELOG.md` updated
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`