fix(compiler): enableLegacyTemplate should not be ignored

[hotforfeature]

Fixes #15555

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
#15555

What is the new behavior?
Setting enableLegacyTemplate: false will no longer cause <template>s to be consumed by the compiler.

The option works correctly, there was just a missing line when merging options from the bootstrap that caused enableLegacyTemplate to always remain at the default true value.

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[X] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[vicb]

Thanks for the PR

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