Add back Gitaly feature flag for cache invalidation

Part of gitaly#2292 (closed)

Related to gitaly#2288 (closed)

What does this MR do?

Upgrades Gitaly to v1.78.0.

One specific thing to note is that this version of Gitaly adds back the cache invalidator feature flag: gitaly!1706 (merged)

See the Gitaly changelog for an overview of all the changes in this release.

This change is essentially a revert of !18766 (merged). Due to an issue found in production (see gitlab-com/gl-infra/production#1501 (comment 262058610)), the cache invalidator needs to be disabled until the cache walker is able to remove empty directories. This feature flag will allow us to reenable the cache invalidator when we are ready to test the updated cache walker.

Note: v1.78.0 fixes the incorrect feature flag string (see gitaly!1715 (merged)).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

This is a low risk change.