Chore: add instant query to query-frontend limitsMiddleware tests

[pracucci]

What this PR does

While working on #8374 we noticed that we don't test limitsMiddleware against instant queries. In this PR I'm doing it. Specifically:

• TestLimitsMiddleware_MaxQueryLookback: create a new TestLimitsMiddleware_MaxQueryLookback_InstantQuery because the assertions for instant queries are different than range queries and remote reads
• TestLimitsMiddleware_MaxQueryExpressionSizeBytes: added instant queries
• TestLimitsMiddleware_MaxQueryLength: doesn't apply to instant queries because we run the check against the input query time range (and instant queries don't have a time range)

Which issue(s) this PR fixes or relates to

N/A

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.

[krajorama]

Let's also add

				"instant query": &PrometheusInstantQueryRequest{
					queryExpr: parseQuery(t, testData.query),
					time:      endMs,
				},

to TestLimitsMiddleware_MaxQueryExpressionSizeBytes

[krajorama]

NitX3: If I understand correctly, start==end for instant query so either we allow it or not, never manipulate it so this is kind of weird name for the testcase.

Suggested change

	"should not manipulate request time if maxQueryLookback and blocksRetentionPeriod are both disabled": {
	"should allow executing a request if maxQueryLookback and blocksRetentionPeriod are both disabled": {

[pracucci]

Yes, absolutely. Going to fix the descriptions in this test.

[pracucci]

Done in 7fac981

[krajorama]

approved with nit