[GC] Remove the option to disable GC mark phase

[agarwal-navin]

GC is enabled by default and has been this way for quite a while. Specifically, the mark phase which marks unreferenced objects as such in the summary is always enabled. This does not effect the container's content or its usage in any way meaning that it doesn't put any restrictions on content retrievel and it doesn't delete anything. These are done by the other two stages of GC - tombstone and sweep. These can still be controlled via settings.

This PR removes the options to turn GC mark phase off. It will always be enabled and cannot be turned off:

• gcAllowed option was used to control the mark phase roll-out. The roll out was completed couple of years ago and has been stable. This option is now removed.
• disableGC option was to provide ability to turn it off in case things went wrong. This was never used (except in tests) and is mow removed.
• The mark phase can still be turned off in tests via the feature flag Fluid.GarbageCollection.Test.RunGC. This used to be called Fluid.GarbageCollection.RunGC and is renamed in this PR to include Test.

[msfluid-bot]

⯆ @fluid-example/bundle-size-tests: -176 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	453.18 KB	453.14 KB	⯆ -44 Bytes
azureClient.js	550.61 KB	550.57 KB	⯆ -44 Bytes
connectionState.js	680 Bytes	680 Bytes	■ No change
containerRuntime.js	257.02 KB	256.98 KB	⯆ -44 Bytes
fluidFramework.js	359.6 KB	359.6 KB	■ No change
loader.js	132.89 KB	132.89 KB	■ No change
map.js	41.45 KB	41.45 KB	■ No change
matrix.js	143.67 KB	143.67 KB	■ No change
odspClient.js	518.94 KB	518.9 KB	⯆ -44 Bytes
odspDriver.js	97.29 KB	97.29 KB	■ No change
odspPrefetchSnapshot.js	42.15 KB	42.15 KB	■ No change
sharedString.js	160.19 KB	160.19 KB	■ No change
sharedTree.js	359.6 KB	359.6 KB	■ No change
Total Size	3.19 MB	3.19 MB	⯆ -176 Bytes

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Baseline commit: 9e9e6e2df7d2681b5f8a2efa39bbad7e56d834e8

Generated by :no_entry_sign: dangerJS against a956071961686e79d0633c36896328ca3f11d328