fix(core): drop vestigial @angular/platform-browser-dynamic peer

[armando-navarro]

ng add @angular/fire fails with npm error ERESOLVE on Angular >= 20.1 apps (#3667).

Cause

1. @angular/fire declares a required peer on @angular/platform-browser-dynamic (^20.0.0 on the v20 line, ^21.0.0 on main).
2. Every platform-browser-dynamic patch release requires the exact @angular/core and @angular/common version matching its own patch (for example, platform-browser-dynamic@20.1.7 declares @angular/core@20.1.7).
3. To satisfy that wide peer range, npm picks some platform-browser-dynamic patch, and that patch in turn requires the exact @angular/core and @angular/common version matching itself (per point 2). That required version usually differs from the core/common the app already has installed, so npm cannot satisfy both at once and the install aborts with ERESOLVE. The reporter's tree pulled in platform-browser-dynamic@20.1.7, which demanded core@20.1.7 while their app had core@20.1.6; a pristine Angular 20 app hits the same conflict through 20.0.7.

Why removing the peer is safe

• Nothing in the published package imports platform-browser-dynamic. The unpacked @angular/fire@20.0.1 and @21.0.0-rc.0 tarballs contain zero references outside the peer declaration itself (checked fesm2022/*.mjs, all .d.ts, and schematics/).
• The repository's only usage is the Karma test bootstrap (src/test.ts), which is supplied by the repository's root package.json and is unaffected.
• Apps that still bootstrap with platformBrowserDynamic() get the package from their own dependencies, never from this peer.
• Angular deprecated platform-browser-dynamic as of 20.1; the peer currently forces a deprecated package into every consumer install.

Alternative considered: marking the peer optional in peerDependenciesMeta (the pattern this manifest already uses for firebase-tools and @angular/platform-server). That would also avoid the ERESOLVE on npm, but it keeps advertising a dependency relationship that does not exist, and the fix would then depend on npm-specific resolver behavior. Removal fixes every package manager. There is precedent for removing unneeded peers outright (#2095).

Verification: built the package from this branch and installed the packed tarball into a fresh Angular 21 app: the install succeeds with no ERESOLVE, and platform-browser-dynamic is absent from the dependency tree. A control install of the currently published package into the same app pulls in the deprecated platform-browser-dynamic@21.2.18.

Angular 20 users get this fix through the planned 20.0.2 patch release, which cherry-picks this commit together with the already-merged config generation fix (#3707).

Refs #3667 (left open deliberately: the reporters there are on the v20 line, so the issue will be closed manually once 20.0.2 ships this fix to npm latest).

[tyler-reitz]

Clean fix, well-justified. Nothing in the published package imports platform-browser-dynamic; the only repo usage is the Karma bootstrap in src/test.ts, which is covered by the root package.json. Removal is the right call over marking it optional.