[operator]: Adapt gardener/dashboard related code

[petersutter]

How to categorize this issue?

/area security /kind enhancement

What would you like to be added: There were some improvements made in the latest gardener/dashboard releases that should be incorporated in the operator:

• [ ] experimentalUseWatchCacheForListShoots setting can be removed (https://github.com/gardener/dashboard/pull/1822)
• [ ] dashboard supports previousSessionSecret, enabling seamless session rotation (https://github.com/gardener/dashboard/pull/1856)
• [x] secrets are mounted as files instead of env vars https://github.com/gardener/gardener/pull/10100
• [ ] oidc.clientSecret is now optional (https://github.com/gardener/dashboard/pull/1835)
• [ ] Lease object is now created during runtime if it does not exist, the corresponding code in the operator can be removed (https://github.com/gardener/dashboard/pull/1823)
• [ ] remove get secret permission for the gardener dashboard. This permission is now obsolete with the ability to display OIDC observability urls (https://github.com/gardener/dashboard/blob/3d692fb2ac6d36af93c5c8bef3aadc42b23d51b1/charts/gardener-dashboard/values.yaml#L236-L239 and https://github.com/gardener/dashboard/pull/1899)

Why is this needed: To adapt to latest (non-breaking) changes introduced with gardener/dashboard 1.75.0 and 1.76.0

[rfranzke]

@petersutter Will you take care of this?

[petersutter]

@grolu wanted to take care

[rfranzke]

/assign @grolu